On Wed, 25 Nov 2020 02:32:34 +0000 Matthew Wilcox <[email protected]> wrote:
> On Tue, Nov 17, 2020 at 11:43:02PM +0000, Matthew Wilcox wrote: > > On Tue, Nov 17, 2020 at 07:15:13PM +0000, Matthew Wilcox wrote: > > > I find both of these functions exceptionally confusing. Does this > > > make it easier to understand? > > > > Never mind, this is buggy. I'll send something better tomorrow. > > That took a week, not a day. *sigh*. At least this is shorter. > > commit 1a02863ce04fd325922d6c3db6d01e18d55f966b > Author: Matthew Wilcox (Oracle) <[email protected]> > Date: Tue Nov 17 10:45:18 2020 -0500 > > fix mm-truncateshmem-handle-truncates-that-split-thps.patch > That's a big patch. Big enough to put mm-truncateshmem-handle-truncates-that-split-thps.patch back into unreviewed state, methinks. And big enough to have a changelog! Below is the folded-together result for reviewers, please. Is the changelog still accurate and complete? From: "Matthew Wilcox (Oracle)" <[email protected]> Subject: mm/truncate,shmem: handle truncates that split THPs Handle THP splitting in the parts of the truncation functions which already handle partial pages. Factor all that code out into a new function called truncate_inode_partial_page(). We lose the easy 'bail out' path if a truncate or hole punch is entirely within a single page. We can add some more complex logic to restore the optimisation if it proves to be worthwhile. [[email protected]: fix] Link: https://lkml.kernel.org/r/[email protected] Link: https://lkml.kernel.org/r/[email protected] Signed-off-by: Matthew Wilcox (Oracle) <[email protected]> Reviewed-by: Jan Kara <[email protected]> Reviewed-by: William Kucharski <[email protected]> Cc: Christoph Hellwig <[email protected]> Cc: Dave Chinner <[email protected]> Cc: Hugh Dickins <[email protected]> Cc: Johannes Weiner <[email protected]> Cc: Kirill A. Shutemov <[email protected]> Cc: Yang Shi <[email protected]> Signed-off-by: Andrew Morton <[email protected]> --- mm/internal.h | 2 mm/shmem.c | 137 +++++++++++++---------------------------- mm/truncate.c | 157 +++++++++++++++++++++++------------------------- 3 files changed, 122 insertions(+), 174 deletions(-) --- a/mm/internal.h~mm-truncateshmem-handle-truncates-that-split-thps +++ a/mm/internal.h @@ -623,4 +623,6 @@ struct migration_target_control { gfp_t gfp_mask; }; +pgoff_t truncate_inode_partial_page(struct address_space *mapping, + struct page *page, loff_t start, loff_t end); #endif /* __MM_INTERNAL_H */ --- a/mm/shmem.c~mm-truncateshmem-handle-truncates-that-split-thps +++ a/mm/shmem.c @@ -858,32 +858,6 @@ void shmem_unlock_mapping(struct address } /* - * Check whether a hole-punch or truncation needs to split a huge page, - * returning true if no split was required, or the split has been successful. - * - * Eviction (or truncation to 0 size) should never need to split a huge page; - * but in rare cases might do so, if shmem_undo_range() failed to trylock on - * head, and then succeeded to trylock on tail. - * - * A split can only succeed when there are no additional references on the - * huge page: so the split below relies upon find_get_entries() having stopped - * when it found a subpage of the huge page, without getting further references. - */ -static bool shmem_punch_compound(struct page *page, pgoff_t start, pgoff_t end) -{ - if (!PageTransCompound(page)) - return true; - - /* Just proceed to delete a huge page wholly within the range punched */ - if (PageHead(page) && - page->index >= start && page->index + HPAGE_PMD_NR <= end) - return true; - - /* Try to split huge page, so we can truly punch the hole or truncate */ - return split_huge_page(page) >= 0; -} - -/* * Remove range of pages and swap entries from page cache, and free them. * If !unfalloc, truncate or punch hole; if unfalloc, undo failed fallocate. */ @@ -892,26 +866,33 @@ static void shmem_undo_range(struct inod { struct address_space *mapping = inode->i_mapping; struct shmem_inode_info *info = SHMEM_I(inode); - pgoff_t start = (lstart + PAGE_SIZE - 1) >> PAGE_SHIFT; - pgoff_t end = (lend + 1) >> PAGE_SHIFT; - unsigned int partial_start = lstart & (PAGE_SIZE - 1); - unsigned int partial_end = (lend + 1) & (PAGE_SIZE - 1); + pgoff_t start, end; struct pagevec pvec; pgoff_t indices[PAGEVEC_SIZE]; + struct page *page; long nr_swaps_freed = 0; pgoff_t index; int i; - if (lend == -1) - end = -1; /* unsigned, so actually very big */ + page = NULL; + start = lstart >> PAGE_SHIFT; + shmem_getpage(inode, start, &page, SGP_READ); + if (page) { + page = thp_head(page); + set_page_dirty(page); + start = truncate_inode_partial_page(mapping, page, lstart, + lend); + } + + /* 'end' includes a partial page */ + end = lend / PAGE_SIZE; pagevec_init(&pvec); index = start; while (index < end && find_lock_entries(mapping, index, end - 1, - &pvec, indices)) { + &pvec, indices)) { for (i = 0; i < pagevec_count(&pvec); i++) { - struct page *page = pvec.pages[i]; - + page = pvec.pages[i]; index = indices[i]; if (xa_is_value(page)) { @@ -921,8 +902,6 @@ static void shmem_undo_range(struct inod index, page); continue; } - index += thp_nr_pages(page) - 1; - if (!unfalloc || !PageUptodate(page)) truncate_inode_page(mapping, page); unlock_page(page); @@ -933,90 +912,60 @@ static void shmem_undo_range(struct inod index++; } - if (partial_start) { - struct page *page = NULL; - shmem_getpage(inode, start - 1, &page, SGP_READ); - if (page) { - unsigned int top = PAGE_SIZE; - if (start > end) { - top = partial_end; - partial_end = 0; - } - zero_user_segment(page, partial_start, top); - set_page_dirty(page); - unlock_page(page); - put_page(page); - } - } - if (partial_end) { - struct page *page = NULL; - shmem_getpage(inode, end, &page, SGP_READ); - if (page) { - zero_user_segment(page, 0, partial_end); - set_page_dirty(page); - unlock_page(page); - put_page(page); - } - } - if (start >= end) - return; - index = start; - while (index < end) { + while (index <= end) { cond_resched(); - if (!find_get_entries(mapping, index, end - 1, &pvec, - indices)) { + if (!find_get_entries(mapping, index, end, &pvec, indices)) { /* If all gone or hole-punch or unfalloc, we're done */ - if (index == start || end != -1) + if (index == start || lend != (loff_t)-1) break; /* But if truncating, restart to make sure all gone */ index = start; continue; } + for (i = 0; i < pagevec_count(&pvec); i++) { - struct page *page = pvec.pages[i]; + page = pvec.pages[i]; - index = indices[i]; if (xa_is_value(page)) { if (unfalloc) continue; - if (shmem_free_swap(mapping, index, page)) { - /* Swap was replaced by page: retry */ - index--; - break; + index = indices[i]; + if (index == end) { + /* Partial page swapped out? */ + shmem_getpage(inode, end, &page, + SGP_READ); + } else { + if (shmem_free_swap(mapping, index, + page)) { + /* Swap replaced: retry */ + break; + } + nr_swaps_freed++; + continue; } - nr_swaps_freed++; - continue; + } else { + lock_page(page); } - lock_page(page); - if (!unfalloc || !PageUptodate(page)) { if (page_mapping(page) != mapping) { /* Page was replaced by swap: retry */ unlock_page(page); - index--; + put_page(page); break; } VM_BUG_ON_PAGE(PageWriteback(page), page); - if (shmem_punch_compound(page, start, end)) - truncate_inode_page(mapping, page); - else if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE)) { - /* Wipe the page and don't get stuck */ - clear_highpage(page); - flush_dcache_page(page); - set_page_dirty(page); - if (index < - round_up(start, HPAGE_PMD_NR)) - start = index + 1; - } + index = truncate_inode_partial_page(mapping, + page, lstart, lend); + if (index > end) + end = indices[i] - 1; } - unlock_page(page); } + index = indices[i - 1] + 1; pagevec_remove_exceptionals(&pvec); - pagevec_release(&pvec); - index++; + pagevec_reinit(&pvec); } spin_lock_irq(&info->lock); --- a/mm/truncate.c~mm-truncateshmem-handle-truncates-that-split-thps +++ a/mm/truncate.c @@ -225,6 +225,63 @@ int truncate_inode_page(struct address_s } /* + * Handle partial (transparent) pages. If the page is entirely within + * the range, we discard it. If not, we split the page if it's a THP + * and zero the part of the page that's within the [start, end] range. + * split_page_range() will discard any of the subpages which now lie + * beyond i_size, and the caller will discard pages which lie within a + * newly created hole. + * + * Return: The index after the current page. + */ +pgoff_t truncate_inode_partial_page(struct address_space *mapping, + struct page *page, loff_t start, loff_t end) +{ + loff_t pos = page_offset(page); + pgoff_t next_index = page->index + thp_nr_pages(page); + unsigned int offset, length; + + if (pos < start) + offset = start - pos; + else + offset = 0; + length = thp_size(page); + if (pos + length <= (u64)end) + length = length - offset; + else + length = end + 1 - pos - offset; + + wait_on_page_writeback(page); + if (length == thp_size(page)) + goto truncate; + + cleancache_invalidate_page(page->mapping, page); + if (page_has_private(page)) + do_invalidatepage(page, offset, length); + if (!PageTransHuge(page)) + goto zero; + page += offset / PAGE_SIZE; + if (split_huge_page(page) < 0) { + page -= offset / PAGE_SIZE; + goto zero; + } + next_index = page->index + 1; + offset %= PAGE_SIZE; + if (offset == 0 && length >= PAGE_SIZE) + goto truncate; + length = PAGE_SIZE - offset; +zero: + zero_user(page, offset, length); + goto out; +truncate: + truncate_inode_page(mapping, page); +out: + unlock_page(page); + put_page(page); + return next_index; +} + +/* * Used to get rid of pages on hardware memory corruption. */ int generic_error_remove_page(struct address_space *mapping, struct page *page) @@ -275,10 +332,6 @@ int invalidate_inode_page(struct page *p * The first pass will remove most pages, so the search cost of the second pass * is low. * - * We pass down the cache-hot hint to the page freeing code. Even if the - * mapping is large, it is probably the case that the final pages are the most - * recently touched, and freeing happens in ascending file offset order. - * * Note that since ->invalidatepage() accepts range to invalidate * truncate_inode_pages_range is able to handle cases where lend + 1 is not * page aligned properly. @@ -286,38 +339,24 @@ int invalidate_inode_page(struct page *p void truncate_inode_pages_range(struct address_space *mapping, loff_t lstart, loff_t lend) { - pgoff_t start; /* inclusive */ - pgoff_t end; /* exclusive */ - unsigned int partial_start; /* inclusive */ - unsigned int partial_end; /* exclusive */ + pgoff_t start, end; struct pagevec pvec; pgoff_t indices[PAGEVEC_SIZE]; pgoff_t index; int i; + struct page * page; if (mapping->nrpages == 0 && mapping->nrexceptional == 0) goto out; - /* Offsets within partial pages */ - partial_start = lstart & (PAGE_SIZE - 1); - partial_end = (lend + 1) & (PAGE_SIZE - 1); + start = lstart >> PAGE_SHIFT; + page = find_lock_head(mapping, start); + if (page) + start = truncate_inode_partial_page(mapping, page, lstart, + lend); - /* - * 'start' and 'end' always covers the range of pages to be fully - * truncated. Partial pages are covered with 'partial_start' at the - * start of the range and 'partial_end' at the end of the range. - * Note that 'end' is exclusive while 'lend' is inclusive. - */ - start = (lstart + PAGE_SIZE - 1) >> PAGE_SHIFT; - if (lend == -1) - /* - * lend == -1 indicates end-of-file so we have to set 'end' - * to the highest possible pgoff_t and since the type is - * unsigned we're using -1. - */ - end = -1; - else - end = (lend + 1) >> PAGE_SHIFT; + /* 'end' includes a partial page */ + end = lend / PAGE_SIZE; pagevec_init(&pvec); index = start; @@ -334,50 +373,11 @@ void truncate_inode_pages_range(struct a cond_resched(); } - if (partial_start) { - struct page *page = find_lock_page(mapping, start - 1); - if (page) { - unsigned int top = PAGE_SIZE; - if (start > end) { - /* Truncation within a single page */ - top = partial_end; - partial_end = 0; - } - wait_on_page_writeback(page); - zero_user_segment(page, partial_start, top); - cleancache_invalidate_page(mapping, page); - if (page_has_private(page)) - do_invalidatepage(page, partial_start, - top - partial_start); - unlock_page(page); - put_page(page); - } - } - if (partial_end) { - struct page *page = find_lock_page(mapping, end); - if (page) { - wait_on_page_writeback(page); - zero_user_segment(page, 0, partial_end); - cleancache_invalidate_page(mapping, page); - if (page_has_private(page)) - do_invalidatepage(page, 0, - partial_end); - unlock_page(page); - put_page(page); - } - } - /* - * If the truncation happened within a single page no pages - * will be released, just zeroed, so we can bail out now. - */ - if (start >= end) - goto out; - index = start; - for ( ; ; ) { + while (index <= end) { cond_resched(); - if (!find_get_entries(mapping, index, end - 1, &pvec, - indices)) { + + if (!find_get_entries(mapping, index, end, &pvec, indices)) { /* If all gone from start onwards, we're done */ if (index == start) break; @@ -387,23 +387,20 @@ void truncate_inode_pages_range(struct a } for (i = 0; i < pagevec_count(&pvec); i++) { - struct page *page = pvec.pages[i]; - - /* We rely upon deletion not changing page->index */ - index = indices[i]; - + page = pvec.pages[i]; if (xa_is_value(page)) continue; lock_page(page); - WARN_ON(page_to_index(page) != index); - wait_on_page_writeback(page); - truncate_inode_page(mapping, page); - unlock_page(page); + index = truncate_inode_partial_page(mapping, page, + lstart, lend); + /* Couldn't split a THP? */ + if (index > end) + end = indices[i] - 1; } + index = indices[i - 1] + 1; truncate_exceptional_pvec_entries(mapping, &pvec, indices); - pagevec_release(&pvec); - index++; + pagevec_reinit(&pvec); } out: _
