linux-kernel - Re: [PATCH RFC 10/17] shmem, userfaultfd: implement shmem uffd operations using vm_uffd

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Message-ID: <aYzf-hS4pUY9ulss@x1.local>
Date: Wed, 11 Feb 2026 15:00:58 -0500
From: Peter Xu <peterx@...hat.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	David Hildenbrand <david@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	James Houghton <jthoughton@...gle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	Michal Hocko <mhocko@...e.com>, Muchun Song <muchun.song@...ux.dev>,
	Nikita Kalyazin <kalyazin@...zon.com>,
	Oscar Salvador <osalvador@...e.de>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Shuah Khan <shuah@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH RFC 10/17] shmem, userfaultfd: implement shmem uffd
 operations using vm_uffd_ops

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.

Thanks,

-- 
Peter Xu