Re: [PATCH v6 4/4] mm/khugepaged: maintain page cache uptodate flag

[Peter Xu <peterx@redhat.com>]

On Tue, Jun 20, 2023 at 01:55:47PM -0700, Andres Freund wrote:
> Hi,

Hi, Andres,

> 
> On 2023-04-04 21:01:17 +0900, David Stevens wrote:
> > From: David Stevens <stevensd@chromium.org>
> > 
> > Make sure that collapse_file doesn't interfere with checking the
> > uptodate flag in the page cache by only inserting hpage into the page
> > cache after it has been updated and marked uptodate. This is achieved by
> > simply not replacing present pages with hpage when iterating over the
> > target range.
> > 
> > The present pages are already locked, so replacing them with the locked
> > hpage before the collapse is finalized is unnecessary. However, it is
> > necessary to stop freezing the present pages after validating them,
> > since leaving long-term frozen pages in the page cache can lead to
> > deadlocks. Simply checking the reference count is sufficient to ensure
> > that there are no long-term references hanging around that would the
> > collapse would break. Similar to hpage, there is no reason that the
> > present pages actually need to be frozen in addition to being locked.
> > 
> > This fixes a race where folio_seek_hole_data would mistake hpage for
> > an fallocated but unwritten page. This race is visible to userspace via
> > data temporarily disappearing from SEEK_DATA/SEEK_HOLE. This also fixes
> > a similar race where pages could temporarily disappear from mincore.
> > 
> > Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> > Signed-off-by: David Stevens <stevensd@chromium.org>
> 
> I noticed that recently MADV_COLLAPSE stopped being able to collapse a
> binary's executable code, always failing with EAGAIN. I bisected it down to
> a2e17cc2efc7 - this commit.
> 
> Using perf trace -e 'huge_memory:*' -a I see
> 
>   1000.433 postgres.2/1872144 huge_memory:mm_khugepaged_collapse_file(mm: 0xffff889e800bdf00, hpfn: 46720000, index: 1537, is_shmem: 1, filename: "postgres.2", result: 17)
>   1000.445 postgres.2/1872144 huge_memory:mm_khugepaged_scan_file(mm: 0xffff889e800bdf00, pfn: -1, filename: "postgres.2", present: 512, result: 17)
>   1000.485 postgres.2/1872144 huge_memory:mm_khugepaged_collapse_file(mm: 0xffff889e800bdf00, hpfn: 46720000, index: 2049, is_shmem: 1, filename: "postgres.2", result: 17)
>   1000.489 postgres.2/1872144 huge_memory:mm_khugepaged_scan_file(mm: 0xffff889e800bdf00, pfn: -1, filename: "postgres.2", present: 512, result: 17)
>   1000.526 postgres.2/1872144 huge_memory:mm_khugepaged_collapse_file(mm: 0xffff889e800bdf00, hpfn: 46720000, index: 2561, is_shmem: 1, filename: "postgres.2", result: 17)
>   1000.532 postgres.2/1872144 huge_memory:mm_khugepaged_scan_file(mm: 0xffff889e800bdf00, pfn: -1, filename: "postgres.2", present: 512, result: 17)
>   1000.570 postgres.2/1872144 huge_memory:mm_khugepaged_collapse_file(mm: 0xffff889e800bdf00, hpfn: 46720000, index: 3073, is_shmem: 1, filename: "postgres.2", result: 17)
>   1000.575 postgres.2/1872144 huge_memory:mm_khugepaged_scan_file(mm: 0xffff889e800bdf00, pfn: -1, filename: "postgres.2", present: 512, result: 17)
> 
> for every attempt at doing madvise(MADV_COLLAPSE).
> 
> 
> I'm sad about that, because MADV_COLLAPSE was the first thing that allowed
> using huge pages for executable code that wasn't entirely completely gross.
> 
> 
> I don't yet have a standalone repro, but can write one if that's helpful.

There's a fix:

https://lore.kernel.org/all/20230607053135.2087354-1-stevensd@google.com/

Already in today's Andrew's pull for rc7:

https://lore.kernel.org/all/20230620123828.813b1140d9c13af900e8edb3@linux-foundation.org/

-- 
Peter Xu