On Mon, Jun 24, 2019 at 05:12:45PM -0700, Song Liu wrote:
> This patch is (hopefully) the first step to enable THP for non-shmem
> filesystems.
>
> This patch enables an application to put part of its text sections to THP
> via madvise, for example:
>
> madvise((void *)0x600000, 0x200000, MADV_HUGEPAGE);
>
> We tried to reuse the logic for THP on tmpfs.
>
> Currently, write is not supported for non-shmem THP. khugepaged will only
> process vma with VM_DENYWRITE. sys_mmap() ignores VM_DENYWRITE requests
> (see ksys_mmap_pgoff). The only way to create vma with VM_DENYWRITE is
> execve(). This requirement limits non-shmem THP to text sections.
>
> The next patch will handle writes, which would only happen when the all
> the vmas with VM_DENYWRITE are unmapped.
>
> An EXPERIMENTAL config, READ_ONLY_THP_FOR_FS, is added to gate this
> feature.
>
> Acked-by: Rik van Riel <[email protected]>
> Signed-off-by: Song Liu <[email protected]>
This is really cool, and less invasive than I anticipated. Nice work.
I only have one concern and one question:
> @@ -1392,6 +1401,29 @@ static void collapse_file(struct mm_struct *mm,
> result = SCAN_FAIL;
> goto xa_unlocked;
> }
> + } else if (!page || xa_is_value(page)) {
> + xas_unlock_irq(&xas);
> + page_cache_sync_readahead(mapping, &file->f_ra, file,
> + index, PAGE_SIZE);
> + /* drain pagevecs to help isolate_lru_page() */
> + lru_add_drain();
> + page = find_lock_page(mapping, index);
> + if (unlikely(page == NULL)) {
> + result = SCAN_FAIL;
> + goto xa_unlocked;
> + }
> + } else if (!PageUptodate(page)) {
> + VM_BUG_ON(is_shmem);
> + xas_unlock_irq(&xas);
> + wait_on_page_locked(page);
> + if (!trylock_page(page)) {
> + result = SCAN_PAGE_LOCK;
> + goto xa_unlocked;
> + }
> + get_page(page);
> + } else if (!is_shmem && PageDirty(page)) {
> + result = SCAN_FAIL;
> + goto xa_locked;
> } else if (trylock_page(page)) {
> get_page(page);
> xas_unlock_irq(&xas);
The many else ifs here check fairly complex page state and are hard to
follow and verify mentally. In fact, it's a bit easier now in the
patch when you see how it *used* to work with just shmem, but the end
result is fragile from a maintenance POV.
The shmem and file cases have little in common - basically only the
trylock_page(). Can you please make one big 'if (is_shmem) {} {}'
structure instead that keeps those two scenarios separate?
> @@ -1426,6 +1458,12 @@ static void collapse_file(struct mm_struct *mm,
> goto out_unlock;
> }
>
> + if (page_has_private(page) &&
> + !try_to_release_page(page, GFP_KERNEL)) {
> + result = SCAN_PAGE_HAS_PRIVATE;
> + break;
> + }
> +
> if (page_mapped(page))
> unmap_mapping_pages(mapping, index, 1, false);
> @@ -1607,6 +1658,17 @@ static void khugepaged_scan_file(struct mm_struct *mm,
> break;
> }
>
> + if (page_has_private(page) && trylock_page(page)) {
> + int ret;
> +
> + ret = try_to_release_page(page, GFP_KERNEL);
> + unlock_page(page);
> + if (!ret) {
> + result = SCAN_PAGE_HAS_PRIVATE;
> + break;
> + }
> + }
> +
> if (page_count(page) != 1 + page_mapcount(page)) {
> result = SCAN_PAGE_COUNT;
> break;
There is already a try_to_release() inside the page lock section in
collapse_file(). I'm assuming you added this one because private data
affects the refcount. But it seems a bit overkill just for that; we
could also still fail the check, in which case we'd have dropped the
buffers in vain. Can you fix the check instead?
There is an is_page_cache_freeable() function in vmscan.c that handles
private fs references:
static inline int is_page_cache_freeable(struct page *page)
{
/*
* A freeable page cache page is referenced only by the caller
* that isolated the page, the page cache and optional buffer
* heads at page->private.
*/
int page_cache_pins = PageTransHuge(page) && PageSwapCache(page) ?
HPAGE_PMD_NR : 1;
return page_count(page) - page_has_private(page) == 1 + page_cache_pins;
}
Wouldn't this work here as well?
The rest looks great to me.
