Re: [PATCH RFC 10/17] shmem, userfaultfd: implement shmem uffd operations using vm_uffd_ops

[Mike Rapoport <rppt@kernel.org>]

On Wed, Feb 11, 2026 at 03:00:58PM -0500, Peter Xu wrote:
> On Sun, Feb 08, 2026 at 12:35:43PM +0200, Mike Rapoport wrote:
> > > > +static void shmem_mfill_filemap_remove(struct folio *folio,
> > > > +				       struct vm_area_struct *vma)
> > > > +{
> > > > +	struct inode *inode = file_inode(vma->vm_file);
> > > > +
> > > > +	filemap_remove_folio(folio);
> > > > +	shmem_recalc_inode(inode, 0, 0);
> > > >  	folio_unlock(folio);
> > > > -	folio_put(folio);
> > > > -out_unacct_blocks:
> > > > -	shmem_inode_unacct_blocks(inode, 1);
> > > 
> > > This looks wrong, or maybe I miss somewhere we did the unacct_blocks()?
> > 
> > This is handled by shmem_recalc_inode(inode, 0, 0).
> 
> IIUC shmem_recalc_inode() only does the fixup of shmem_inode_info over
> possiblly changing inode->i_mapping->nrpages.  It's not for reverting the
> accounting in the failure paths here.
> 
> OTOH, we still need to maintain accounting for the rest things with
> correctly invoke shmem_inode_unacct_blocks().  One thing we can try is
> testing this series against either shmem quota support (since 2023, IIUC
> it's relevant to "quota" mount option), or max_blocks accountings (IIUC,
> "size" mount option), etc.  Any of those should reflect a difference if my
> understanding is correct.
> 
> So IIUC we still need the unacct_blocks(), please kindly help double check.

I followed shmem_get_folio_gfp() error handling, and unless I missed
something we should have the same sequence with uffd.

In shmem_mfill_filemap_add() we increment both i_mapping->nrpages and
info->alloced in shmem_add_to_page_cache() and 
shmem_recalc_inode(inode, 1, 0) respectively.

Then in shmem_filemap_remove() the call to filemap_remove_folio()
decrements i_mapping->nrpages and shmem_recalc_inode(inode, 0, 0) will see
freed=1 and will call shmem_inode_unacct_blocks().
 
> Thanks,
> 
> -- 
> Peter Xu
> 

-- 
Sincerely yours,
Mike.