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

[David Hildenbrand (Arm)]

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 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 :)

-- 
Cheers,

David