Re: [PATCH v1 2/2] fuse: use iomap helpers to update folio uptodate state

On Wed, Jun 24, 2026 at 04:29:54AM +0100, Matthew Wilcox wrote:
> I believe this to be a bug.  I talked about this a bit as LSFMM, but the
> fundamental problem is that the folio uptodate flag DOES NOT BELONG TO THE
> FILESYSTEM (unlike the buffer_head uptodate flag, which definitely does).
> 
> The MM has a number of sanity checks that make sure we haven't
> done something awful like mapping a !uptodate folio into userspace.
> That would potentially leak confidential information to userspace.
> So when a filesystem clears the uptodate flag, the MM can come along
> and bring down the whole machine because the mapped folio is now !uptodate.
> 
> I know we have a terrible writeback error handling story.  But I think
> the right thing to do is just take out the calls to clear uptodate.
> In all filesystems.  Darrick took it out of iomap in e9c3a8e820ed but
> it's time to purge it everywhere.

Yeah.  We're not going to fix everything at once, but we should
slowly move to stop clearing the folio uptodate bit.  And I suspect
the same is trye for the sub-folio uptodate bits.