Re: [PATCH 00/16] mm/shmem: various (urgent?) fixes

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 10/26/26 5:40 PM, David Hildenbrand (Arm) wrote:
> Hi,
> 
> the fixing fun continues.
> 
> This is a handful of fixes for shmem/tmpfs. I sneaked in a related
> hugetlb fix that stalled upstream.

Wow, thanks David!

> Some of the stuff is more urgent (patch #1, F_SEAL_EXEC, truncation
> hangs), others are less urgent but still rather unpleasant.
> 
> The majority are fixes from Anthropic that I cleaned up or reworked.
> While doing that, I found a bunch of other issues and tried to fix them
> somewhat cleanly.
> 
> In general, shmem + swap handling is a nightmare. shmem_undo_range() is
> such utter garbage it makes me want to cry. I mean, really, WTF did we
> end up creating there. Does any expert here have time to clean that up and
> make it *actually* maintainable.

Agree. :) I'll re-read shmem_undo_range() and take a close look at the 
issues you raised, and see if I can make shmem_undo_range() more readable.

> The following things are still broken in shmem_undo_range():
> 
> (1) Truncate/punch of a single page of a large swap-error marker
> 
> We don't split. Consequently, read access afterwards will still fail
> instead of returning 0s.
> 
> I have a fix to try to split (which can fail), but I don't consider it
> that urgent, really. So I'm not including it. Might send it later
> separately.
> 
> Truncate/punch of a partial page can obviously never do the right thing,
> as we cannot poison parts of pages ...
> 
> (2) Truncate/punch whole pages within part of a large swap entry
> 
> We try to read the entry, but if we are out of memory that might fail.
> Consequently, we will not free up swap space and a re-read will not give
> us 0s but the previous content.
> 
> AI proposed a nasty NOFAIL patch which has too many nasty side effects
> IMHO (allowing users to force-read pages and retain them afterwards even
> though memory.max would disallow it). This needs a proper thought. Likely,
> support for splitting large swap entries (and large folios in the
> swapcache, which I recall is under development  by Kairui). But splitting
> can fail ...
> 
> (3) Truncate/punch of a part of a base-page swap entry
> 
> We obviously have to read the page in, in order to 0-out the parts to
> discard. That might obviously fail.
> 
> Maybe we should install swapin-error markers if swapping during
> truncate/punchhole fails, to at least tell user space on later access
> that something went wrong? We might affect unrelated data, outside the
> requested range, though. Anyhow, these are long standing issues and
> I don't consider them urgent.
> 
> They need a proper solution.
> 
> I did my best trying to test these fixes. I was able to reproduce some
> of the things (including unkillable truncation hangs), and the fixes seemed
> to help.
> 
> But again, shmem_undo_range() is just an absolute mess.
> 
> Please prioritize review of these fixes, I want to get fixes for the
> issues in a timely manner upstream.

OK. Will review and test this serires this week.

> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> ---
> Chris Mason (5):
>        mm/mempolicy: charge shared policy nodes
>        mm/shmem: honour F_SEAL_EXEC when an ACL is set
>        mm/shmem: avoid moving the truncation start backwards
>        mm/shmem: stop retrying folios that cannot be split
>        mm/shmem: preserve data in clean folios when splitting fails
> 
> David Hildenbrand (Arm) (7):
>        mm/shmem: discard whole new large folios on fallocate rollback
>        mm/shmem: rescan after nonuniform folio splitting
>        mm/shmem: fix truncate hang in large swap entry
>        mm/shmem: preserve allocation errors when swapping in large entries
>        mm/shmem: fix truncate loop after boundary swapin failure
>        mm/shmem: fix accounting of swapin-error markers
>        mm/shmem: fix truncation of files containing only swapin-error markers
> 
> Eva Kurchatova (1):
>        hugetlbfs: enforce F_SEAL_EXEC in setattr
> 
> Josef Bacik (3):
>        tmpfs: keep the quota soft limits of an unused id
>        tmpfs: preserve explicitly disabled quota limits
>        mm/shmem: hold off eviction in shmem_writeout()
> 
>   fs/hugetlbfs/inode.c  |   8 +++
>   include/linux/memfd.h |   9 +++
>   mm/mempolicy.c        |   4 +-
>   mm/shmem.c            | 170 +++++++++++++++++++++++++++++++++++---------------
>   mm/shmem_quota.c      |  28 +++++----
>   5 files changed, 154 insertions(+), 65 deletions(-)
> 
> ---
> 
> base-commit: 21a7f44fd0765fcc5194130b28767786eba1d1e7
> 
> change-id: 20261025-fixes_shmem-2782719b130e
> 
> --
> 
> Cheers,
> 
> David