Re: [PATCH 07/16] mm/shmem: avoid moving the truncation start backwards

[Zi Yan]

On 26 Oct 2026, at 19:24, David Hildenbrand (Arm) wrote:

> On 10/27/26 00:16, Zi Yan wrote:
>> On 26 Oct 2026, at 18:39, Zi Yan wrote:
>>
>>> On 26 Oct 2026, at 17:13, David Hildenbrand (Arm) wrote:
>>>
>>>>
>>>> How confident are we that it fixes the same issue?
>>>
>>> The issue here is that folio_next_index(folio) used in the mainline
>>> code can be an index lower than lstart and the fix is use max()
>>> to prevent start going backwards. (I wrote this to make sure
>>> I understand the issue correctly)
>>>
>>> Zhang Yi’s patch passes start into truncate_inode_parital_folio(),
>>> which has a new signature, and it is updated with the folio’s index
>>> within the [lstart, lend] range at the beginning.
>>>
>>> So this patch fixes the issue at the caller and Zhang Yi’s patch
>>> fixes the issue inside truncate_inode_partial_folio().
>>>
>>>>
>>>> Does it also cover #8?
>>>
>>> No, patch 8 is still needed. I am checking it on how to rebase it
>>> on mm-stable.
>>>
>>> Patch 9, 10, 11 might need rebase on mm-stable too.
>>>
>>> Since these patches are on top of mainline instead of mm-stable,
>>> you will need to rebase them. IIRC, Zhang Yi’s patches are needed
>>> for ext4 + iomap buffered I/O conversion. Hmm, but Zhang Yi’s
>>> ext4 iomap patches has landed in mainline but its dependency
>>> is not. So we cannot drop Zhang Yi’s patches and ask for a
>>> resend. Adding Zhang Yi here.
>>
>> To clarify the dependency, without Zhang Yi’s
>> truncate_inode_partial_folio() patchset, the ext4 + iomap changes
>> would cause data loss. We probably want to get Zhang Yi’s patches
>> in first to avoid that.
>
> Ugh, what a mess.
>
>>
>> Patch 7 to 11 do not apply cleanly on top of mm-stable due to Zhang Yi’s
>> truncate fix, where
>>
>> 1. patch 7 can be dropped as I discussed above,
>> 2. patch 8 needs a folio->mapping guard for the added statement,
>> 3. patch 9-11 can be applied with fuzzy, but I need to look into
>> them about functionalities before fully trust what LLM says.
> Yeah, let me take a look tomorrow.

I will reply to these patches individually in terms of rebasing on
top of mm-stable and how they interact with Zhang Yi’s changes.
It is not too bad at first look.

>
> I cannot put into words how much this shmem + pagecache shite annoys me.
>
> What a piece of crap people created here. And looking at the massive changes to
> fix stuff, what a piece of crap people are creating.
>
> I better go to sleep now :)

Good night. :)

Best Regards,
Yan, Zi