From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Yang Shi <shy828301@gmail.com>
Cc: naoya.horiguchi@nec.com, hughd@google.com,
kirill.shutemov@linux.intel.com, willy@infradead.org,
osalvador@suse.de, akpm@linux-foundation.org, linux-mm@kvack.org,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] mm: khugepaged: check if file page is on LRU after locking page
Date: Wed, 15 Sep 2021 14:49:47 +0300 [thread overview]
Message-ID: <20210915114947.2zh7inouztenth6o@box.shutemov.name> (raw)
In-Reply-To: <20210914183718.4236-3-shy828301@gmail.com>
On Tue, Sep 14, 2021 at 11:37:16AM -0700, Yang Shi wrote:
> The khugepaged does check if the page is on LRU or not but it doesn't
> hold page lock. And it doesn't check this again after holding page
> lock. So it may race with some others, e.g. reclaimer, migration, etc.
> All of them isolates page from LRU then lock the page then do something.
>
> But it could pass the refcount check done by khugepaged to proceed
> collapse. Typically such race is not fatal. But if the page has been
> isolated from LRU before khugepaged it likely means the page may be not
> suitable for collapse for now.
>
> The other more fatal case is the following patch will keep the poisoned
> page in page cache for shmem, so khugepaged may collapse a poisoned page
> since the refcount check could pass. 3 refcounts come from:
> - hwpoison
> - page cache
> - khugepaged
>
> Since it is not on LRU so no refcount is incremented from LRU isolation.
>
> This is definitely not expected. Checking if it is on LRU or not after
> holding page lock could help serialize against hwpoison handler.
>
> But there is still a small race window between setting hwpoison flag and
> bump refcount in hwpoison handler. It could be closed by checking
> hwpoison flag in khugepaged, however this race seems unlikely to happen
> in real life workload. So just check LRU flag for now to avoid
> over-engineering.
>
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
> mm/khugepaged.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 045cc579f724..bdc161dc27dc 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1808,6 +1808,12 @@ static void collapse_file(struct mm_struct *mm,
> goto out_unlock;
> }
>
> + /* The hwpoisoned page is off LRU but in page cache */
> + if (!PageLRU(page)) {
> + result = SCAN_PAGE_LRU;
> + goto out_unlock;
> + }
> +
> if (isolate_lru_page(page)) {
isolate_lru_page() should catch the case, no? TestClearPageLRU would fail
and we get here.
> result = SCAN_DEL_PAGE_LRU;
> goto out_unlock;
> --
> 2.26.2
>
>
--
Kirill A. Shutemov
next prev parent reply other threads:[~2021-09-15 11:49 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-14 18:37 [RFC PATCH 0/4] Solve silent data loss caused by poisoned page cache (shmem/tmpfs) Yang Shi
2021-09-14 18:37 ` [PATCH 1/4] mm: filemap: check if any subpage is hwpoisoned for PMD page fault Yang Shi
2021-09-15 11:46 ` Kirill A. Shutemov
2021-09-15 17:28 ` Yang Shi
2021-09-14 18:37 ` [PATCH 2/4] mm: khugepaged: check if file page is on LRU after locking page Yang Shi
2021-09-15 11:49 ` Kirill A. Shutemov [this message]
2021-09-15 17:48 ` Yang Shi
2021-09-15 23:00 ` Yang Shi
2021-09-15 23:10 ` Yang Shi
2021-09-14 18:37 ` [PATCH 3/4] mm: shmem: don't truncate page if memory failure happens Yang Shi
2021-09-21 9:49 ` Naoya Horiguchi
2021-09-21 19:34 ` Yang Shi
2021-09-14 18:37 ` [PATCH 4/4] mm: hwpoison: handle non-anonymous THP correctly Yang Shi
2021-09-21 9:50 ` Naoya Horiguchi
2021-09-21 19:46 ` Yang Shi
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