On Mon, 12 Mar 2018 17:42:17 +0100 Claudio Imbrenda <[email protected]> wrote:
> This patch fixes a corner case for KSM. When two pages belong or > belonged to the same transparent hugepage, and they should be merged, > KSM fails to split the page, and therefore no merging happens. > > This bug can be reproduced by: > * making sure ksm is running (in case disabling ksmtuned) > * enabling transparent hugepages > * allocating a THP-aligned 1-THP-sized buffer > e.g. on amd64: posix_memalign(&p, 1<<21, 1<<21) > * filling it with the same values > e.g. memset(p, 42, 1<<21) > * performing madvise to make it mergeable > e.g. madvise(p, 1<<21, MADV_MERGEABLE) > * waiting for KSM to perform a few scans > > The expected outcome is that the all the pages get merged (1 shared and > the rest sharing); the actual outcome is that no pages get merged (1 > unshared and the rest volatile) > > The reason of this behaviour is that we increase the reference count > once for both pages we want to merge, but if they belong to the same > hugepage (or compound page), the reference counter used in both cases > is the one of the head of the compound page. > This means that split_huge_page will find a value of the reference > counter too high and will fail. > > This patch solves this problem by testing if the two pages to merge > belong to the same hugepage when attempting to merge them. If so, the > hugepage is split safely. This means that the hugepage is not split if > not necessary. > > Signed-off-by: Gerald Schaefer <[email protected]> > Signed-off-by: Claudio Imbrenda <[email protected]> Signoff trail is confusing. Usually people put the primary author's signoff first, which makes me wonder whether you or Gerald was the primary author? > diff --git a/mm/ksm.c b/mm/ksm.c > index 293721f..7a826fa 100644 > --- a/mm/ksm.c > +++ b/mm/ksm.c > @@ -2001,7 +2001,7 @@ static void cmp_and_merge_page(struct page *page, > struct rmap_item *rmap_item) > struct page *kpage; > unsigned int checksum; > int err; > - bool max_page_sharing_bypass = false; > + bool split, max_page_sharing_bypass = false; `split' could be made local to the `if' block where it is used, which improves readability and maintainability somewhat. > stable_node = page_stable_node(page); > if (stable_node) { > @@ -2084,6 +2084,8 @@ static void cmp_and_merge_page(struct page *page, > struct rmap_item *rmap_item) > if (tree_rmap_item) { > kpage = try_to_merge_two_pages(rmap_item, page, > tree_rmap_item, tree_page); > + split = PageTransCompound(page) && PageTransCompound(tree_page) > + && compound_head(page) == compound_head(tree_page); I think a comment explainig what's going on would be useful here. > put_page(tree_page); > if (kpage) { > /* > @@ -2110,6 +2112,11 @@ static void cmp_and_merge_page(struct page *page, > struct rmap_item *rmap_item) > break_cow(tree_rmap_item); > break_cow(rmap_item); > } > + } else if (split) { > + if (!trylock_page(page)) > + return; > + split_huge_page(page); > + unlock_page(page); Why did we use trylock_page()? Perhaps for the same reasons which were explained in try_to_merge_one_page(), perhaps for other reasons. cmp_and_merge_page() already does lock_page() and down_read(), so I wonder if those reasons are legitimate. Again, a comment here is needed - otherwise it will be hard for readers to understand your intent. > } > } > }
