Re: [PATCH v2] tmpfs: fault in smaller chunks if large folio allocation not allowed

[Matthew Wilcox <willy@infradead.org>]

On Thu, Sep 26, 2024 at 10:20:54PM +0800, Kefeng Wang wrote:
> On 2024/9/26 21:52, Matthew Wilcox wrote:
> > On Thu, Sep 26, 2024 at 10:38:34AM +0200, Pankaj Raghav (Samsung) wrote:
> > > > So this is why I don't use mapping_set_folio_order_range() here, but
> > > > correct me if I am wrong.
> > > 
> > > Yeah, the inode is active here as the max folio size is decided based on
> > > the write size, so probably mapping_set_folio_order_range() will not be
> > > a safe option.
> > 
> > You really are all making too much of this.  Here's the patch I think we
> > need:
> > 
> > -       mapping_set_large_folios(inode->i_mapping);
> > +       if (sbinfo->huge)
> > +               mapping_set_large_folios(inode->i_mapping);
> 
> But it can't solve all issue, eg,
>   mount with huge = SHMEM_HUGE_WITHIN_SIZE, or

The page cache will not create folios which overhang the end of the file
by more than the minimum folio size for that mapping.  So this is wrong.

>   mount with SHMEM_HUGE_ALWAYS  +  runtime SHMEM_HUGE_DENY

That's a tweak to this patch, not a fundamental problem with it.

> and the above change will break
>   mount with SHMEM_HUGE_NEVER + runtime SHMEM_HUGE_FORCE

Likewise.