Re: [PATCH 5/6] iomap: drop unnecessary state_lock when setting ifs uptodate bits
From: Zhang Yi
Date: Wed Aug 07 2024 - 07:39:30 EST
On 2024/8/5 23:48, Matthew Wilcox wrote:
> On Mon, Aug 05, 2024 at 04:00:23PM +0200, Jan Kara wrote:
>> Actually add Matthew to CC ;)
>
> It's OK, I was reading.
>
> FWIW, I agree with Dave; the locking complexity in this patch was
> horrendous. I was going to get to the same critique he had, but I first
> wanted to understand what the thought process was.
Yes, I'd like to change to use the solution as Dave suggested.
>
>>>> Ha, right, I missed the comments of this function, it means that there are
>>>> some special callers that hold table lock instead of folio lock, is it
>>>> pte_alloc_map_lock?
>>>>
>>>> I checked all the filesystem related callers and didn't find any real
>>>> caller that mark folio dirty without holding folio lock and that could
>>>> affect current filesystems which are using iomap framework, it's just
>>>> a potential possibility in the future, am I right?
>
> Filesystems are normally quite capable of taking the folio lock to
> prevent truncation. It's the MM code that needs the "or holding the
> page table lock" get-out clause. I forget exactly which callers it
> is; I worked through them a few times. It's not hard to put a
> WARN_ON_RATELIMIT() into folio_mark_dirty() and get a good sampling.
>
> There's also a "or holding a buffer_head locked" get-out clause that
> I'm not sure is documented anywhere, but obviously that doesn't apply
> to the iomap code.
Thanks for your answer, I've found some callers.
Thanks,
Yi.
>
>>> There used to be quite a few places doing that. Now that I've checked all
>>> places I was aware of got actually converted to call folio_mark_dirty() under
>>> a folio lock (in particular all the cases happening on IO completion, folio
>>> unmap etc.). Matthew, are you aware of any place where folio_mark_dirty()
>>> would be called for regular file page cache (block device page cache is in a
>>> different situation obviously) without folio lock held?
>
> Yes, the MM code definitely applies to regular files as well as block
> devices.
>