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

[Andres Freund]

Hi,

On 2023-04-04 21:01:17 +0900, David Stevens wrote:
> From: David Stevens <stevensd@xxxxxxxxxxxx>
> 
> 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@xxxxxxxxxxxx>

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.

Greetings,

Andres Freund