Re: [PATCH RESEND V2 0/5] Fixes and cleanups to xarray

[Kemeng Shi <shikemeng@huaweicloud.com>]

on 11/12/2024 5:28 AM, Andrew Morton wrote:
> On Tue, 12 Nov 2024 05:53:54 +0800 Kemeng Shi <shikemeng@huaweicloud.com> wrote:
> 
>> This series contains some random fixes and cleanups to xarray. Patch 1-3
>> are fixes and patch 4-5 are cleanups. More details can be found in
>> respective patches. 
> 
> As we're at -rc7 I'm looking to stash low-priority things away for
> consideration after 6.13-r1.
> 
> But I don't know if all of these are low-priority things.  Because you
> didn't tell me!
> 
> Please please always always describe the possible userspace-visible effects of
> all fixes.  And all non-fixes, come to that.>
> So.  Please go through all of the patches in this series and redo the
> changelogs, describing the impact each patch might have for our users.
Sorry for missing these information, I will add detail of impact to userspace
in next version.
As there are a lot of users of xarray in kernel and I'm not familar to
most of them, I maybe not able to send new series with description of
userspace-visible effects in time. I'd like to describe effect to users
of xarray in kernel here to give some early inputs.

Patch 1 fixes the issue that xas_find_marked() will return a sibling entry
to users when there is a race to replace old entry with small range with
a new entry with bigger range. The kernel will crash if users use returned
sibling entry as a valid entry.
For example, find_get_entry() will crash if it gets a sibling entry from
xas_find_marked() when trying to search a writeback folios.

Patch 2 fixes the issue that xas_split_alloc() doesn't distinguish large
entries whose order is == xas->xa_shift + 2 * XA_CHUNK_SHIFT in whice case
the splited entries have bigger order than expected. Then afterward
xas_store will assign stored entry with bigger range than expected and we
will get wrong entry from slot after actual range of stored entry. Not
sure if we will split large entry with order == xas->xa_shift + 2 *
XA_CHUNK_SHIFT.

Patch 3 fixes the issue that xas_pause() may skip some entry unepxectedly
after xas_load()(may be called in xas_for_each for first entry) loads a
large entry with index point at mid of the entry. So we may miss to writeback
some dirty page and lost user data.

> 
> If any of these patches should, in your opinion, be included in 6.12
> and/or backported into -stable trees then it would be better to
> separate them into a different patch series.  But a single patch series
> is acceptable - I can figure this stuff out, if I'm given the necessary
> information!  
As impact descripted above, patch 1-3 should be include in 6.12 and backported
into -stable trees. But we need to confirm if the trigger condition exists
in users(filemap, shmem and so on) of xarray in real world. Will include more
information in next version.

Thanks.
> 
> Thanks.
>