Re: [PATCH 05/15] fs: afs: correctly drop reference count on mapping failure

Lorenzo Stoakes (Oracle)
2026-03-13T12:00:19Z

On Fri, Mar 13, 2026 at 04:07:43AM -0700, Usama Arif wrote:
> On Thu, 12 Mar 2026 20:27:20 +0000 "Lorenzo Stoakes (Oracle)" <ljs@kernel.org> wrote:
>
> > Commit 9d5403b1036c ("fs: convert most other generic_file_*mmap() users to
> > .mmap_prepare()") updated AFS to use the mmap_prepare callback in favour of
> > the deprecated mmap callback.
> >
> > However, it did not account for the fact that mmap_prepare can fail to map
> > due to an out of memory error, and thus should not be incrementing a
> > reference count on mmap_prepare.
> >
> > With the newly added vm_ops->mapped callback available, we can simply defer
> > this operation to that callback which is only invoked once the mapping is
> > successfully in place (but not yet visible to userspace as the mmap and VMA
> > write locks are held).
> >
> > Therefore add afs_mapped() to implement this callback for AFS.
> >
> > In practice the mapping allocations are 'too small to fail' so this is
> > something that realistically should never happen in practice (or would do
> > so in a case where the process is about to die anyway), but we should still
> > handle this.
> >
> > Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> > ---
> > fs/afs/file.c | 20 ++++++++++++++++----
> > 1 file changed, 16 insertions(+), 4 deletions(-)
> >
> > diff --git a/fs/afs/file.c b/fs/afs/file.c
> > index f609366fd2ac..69ef86f5e274 100644
> > --- a/fs/afs/file.c
> > +++ b/fs/afs/file.c
> > @@ -28,6 +28,8 @@ static ssize_t afs_file_splice_read(struct file *in, loff_t *ppos,
> > static void afs_vm_open(struct vm_area_struct *area);
> > static void afs_vm_close(struct vm_area_struct *area);
> > static vm_fault_t afs_vm_map_pages(struct vm_fault *vmf, pgoff_t start_pgoff, pgoff_t end_pgoff);
> > +static int afs_mapped(unsigned long start, unsigned long end, pgoff_t pgoff,
> > + const struct file *file, void **vm_private_data);
> >
> > const struct file_operations afs_file_operations = {
> > .open = afs_open,
> > @@ -61,6 +63,7 @@ const struct address_space_operations afs_file_aops = {
> > };
> >
> > static const struct vm_operations_struct afs_vm_ops = {
> > + .mapped = afs_mapped,
> > .open = afs_vm_open,
> > .close = afs_vm_close,
> > .fault = filemap_fault,
> > @@ -500,13 +503,22 @@ static int afs_file_mmap_prepare(struct vm_area_desc *desc)
> > afs_add_open_mmap(vnode);
>
> Is the above afs_add_open_mmap an additional one, which could cause a reference
> leak? Does the above one need to be removed and only the one in afs_mapped()
> needs to be kept?

Ah yeah good spot, will fix thanks!

>
> >
> > ret = generic_file_mmap_prepare(desc);
> > - if (ret == 0)
> > - desc->vm_ops = &afs_vm_ops;
> > - else
> > - afs_drop_open_mmap(vnode);
> > + if (ret)
> > + return ret;
> > +
> > + desc->vm_ops = &afs_vm_ops;
> > return ret;
> > }
> >
> > +static int afs_mapped(unsigned long start, unsigned long end, pgoff_t pgoff,
> > + const struct file *file, void **vm_private_data)
> > +{
> > + struct afs_vnode *vnode = AFS_FS_I(file_inode(file));
> > +
> > + afs_add_open_mmap(vnode);
> > + return 0;
> > +}
> > +
> > static void afs_vm_open(struct vm_area_struct *vma)
> > {
> > afs_add_open_mmap(AFS_FS_I(file_inode(vma->vm_file)));
> > --
> > 2.53.0
> >
> >

Cheers, Lorenzo