Re: [PATCH v8 14/18] mm: memfd_luo: allow preserving memfd

[Chris Mason <clm@meta.com>]

On Tue, 25 Nov 2025 11:58:44 -0500 Pasha Tatashin <pasha.tatashin@soleen.com> wrote:

Hi everyone,

I'm running the v6.19 mm commits through patch review automation, and this
commit was flagged.  I don't know this code well, but it looks like it
might be a real bug.

[AI review output below]

> diff --git a/mm/memfd_luo.c b/mm/memfd_luo.c
> new file mode 100644
> index 0000000000000..4f6ba63b43105
> --- /dev/null
> +++ b/mm/memfd_luo.c

[ ... ]

> +static int memfd_luo_retrieve_folios(struct file *file,
> +				     struct memfd_luo_folio_ser *folios_ser,
> +				     u64 nr_folios)
> +{
> +	struct inode *inode = file_inode(file);
> +	struct address_space *mapping = inode->i_mapping;
> +	struct folio *folio;
> +	int err = -EIO;
> +	long i;
> +
> +	for (i = 0; i < nr_folios; i++) {

[ ... ]

> +		err = shmem_add_to_page_cache(folio, mapping, index, NULL,
> +					      mapping_gfp_mask(mapping));
> +		if (err) {
> +			pr_err("shmem: failed to add to page cache folio index %ld: %d\n",
> +			       i, err);
> +			goto unlock_folio;
> +		}
> +
> +		if (flags & MEMFD_LUO_FOLIO_UPTODATE)
> +			folio_mark_uptodate(folio);
> +		if (flags & MEMFD_LUO_FOLIO_DIRTY)
> +			folio_mark_dirty(folio);
> +
> +		err = shmem_inode_acct_blocks(inode, 1);
> +		if (err) {
> +			pr_err("shmem: failed to account folio index %ld: %d\n",
> +			       i, err);
> +			goto unlock_folio;
> +		}

When shmem_inode_acct_blocks() fails here, the folio has already been
added to the page cache by shmem_add_to_page_cache(). Should the folio be
removed from the page cache before going to unlock_folio?

Looking at shmem_alloc_and_add_folio() in mm/shmem.c, when
shmem_inode_acct_blocks() fails after the folio has been added, it calls
filemap_remove_folio() to remove it:

    error = shmem_inode_acct_blocks(inode, pages);
    if (error) {
        ...
        if (error) {
            filemap_remove_folio(folio);
            goto unlock;
        }
    }

Without this, the folio remains in the page cache (counted in
mapping->nrpages) but info->alloced is not incremented (since
shmem_recalc_inode is not called). This could cause shmem accounting
inconsistency.