From: David Hildenbrand <david@xxxxxxxxxx> Subject: mm: slightly clarify KSM logic in do_swap_page() Let's make it clearer that KSM might only have to copy a page in case we have a page in the swapcache, not if we allocated a fresh page and bypassed the swapcache. While at it, add a comment why this is usually necessary and merge the two swapcache conditions. [akpm@xxxxxxxxxxxxxxxxxxxx: fix comment, per David] Link: https://lkml.kernel.org/r/20220131162940.210846-4-david@xxxxxxxxxx Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> Cc: Christoph Hellwig <hch@xxxxxx> Cc: David Rientjes <rientjes@xxxxxxxxxx> Cc: Don Dutile <ddutile@xxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Jan Kara <jack@xxxxxxx> Cc: Jann Horn <jannh@xxxxxxxxxx> Cc: Jason Gunthorpe <jgg@xxxxxxxxxx> Cc: John Hubbard <jhubbard@xxxxxxxxxx> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Liang Zhang <zhangliang5@xxxxxxxxxx> Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx> Cc: Nadav Amit <nadav.amit@xxxxxxxxx> Cc: Oleg Nesterov <oleg@xxxxxxxxxx> Cc: Peter Xu <peterx@xxxxxxxxxx> Cc: Rik van Riel <riel@xxxxxxxxxxx> Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx> Cc: Shakeel Butt <shakeelb@xxxxxxxxxx> Cc: Yang Shi <shy828301@xxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/memory.c | 36 ++++++++++++++++++++++-------------- 1 file changed, 22 insertions(+), 14 deletions(-) --- a/mm/memory.c~mm-slightly-clarify-ksm-logic-in-do_swap_page +++ a/mm/memory.c @@ -3607,21 +3607,29 @@ vm_fault_t do_swap_page(struct vm_fault goto out_release; } - /* - * Make sure try_to_free_swap or reuse_swap_page or swapoff did not - * release the swapcache from under us. The page pin, and pte_same - * test below, are not enough to exclude that. Even if it is still - * swapcache, we need to check that the page's swap has not changed. - */ - if (unlikely((!PageSwapCache(page) || - page_private(page) != entry.val)) && swapcache) - goto out_page; + if (swapcache) { + /* + * Make sure try_to_free_swap or swapoff did not release the + * swapcache from under us. The page pin, and pte_same test + * below, are not enough to exclude that. Even if it is still + * swapcache, we need to check that the page's swap has not + * changed. + */ + if (unlikely(!PageSwapCache(page) || + page_private(page) != entry.val)) + goto out_page; - page = ksm_might_need_to_copy(page, vma, vmf->address); - if (unlikely(!page)) { - ret = VM_FAULT_OOM; - page = swapcache; - goto out_page; + /* + * KSM sometimes has to copy on read faults, for example, if + * page->index of !PageKSM() pages would be nonlinear inside the + * anon VMA -- PageKSM() is lost on actual swapout. + */ + page = ksm_might_need_to_copy(page, vma, vmf->address); + if (unlikely(!page)) { + ret = VM_FAULT_OOM; + page = swapcache; + goto out_page; + } } cgroup_throttle_swaprate(page, GFP_KERNEL); _
