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

[Andres Freund]

Hi,

On 2023-06-20 17:11:30 -0400, Peter Xu wrote:
> On Tue, Jun 20, 2023 at 01:55:47PM -0700, Andres Freund wrote:
> > 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.
> 
> There's a fix:
> 
> https://lore.kernel.org/all/20230607053135.2087354-1-stevensd@xxxxxxxxxx/
> 
> Already in today's Andrew's pull for rc7:
> 
> https://lore.kernel.org/all/20230620123828.813b1140d9c13af900e8edb3@xxxxxxxxxxxxxxxxxxxx/

Ah, great!

I can confirm that the fix unbreaks our use of MADV_COLLAPSE for executable
code...

Greetings,

Andres Freund