Re: [PATCHv2 2/2] mm/truncate: Unmap large folio on split failure

[Kiryl Shutsemau]

On Wed, Oct 29, 2025 at 08:19:47AM -0700, Darrick J. Wong wrote:
> On Wed, Oct 29, 2025 at 10:21:53AM +0000, Kiryl Shutsemau wrote:
> > On Wed, Oct 29, 2025 at 02:12:48AM -0700, Hugh Dickins wrote:
> > > On Mon, 27 Oct 2025, Kiryl Shutsemau wrote:
> > > > On Mon, Oct 27, 2025 at 03:10:29AM -0700, Hugh Dickins wrote:
> > > ...
> > > > 
> > > > > Aside from shmem/tmpfs, it does seem to me that this patch is
> > > > > doing more work than it needs to (but how many lines of source
> > > > > do we want to add to avoid doing work in the failed split case?):
> > > > > 
> > > > > The intent is to enable SIGBUS beyond EOF: but the changes are
> > > > > being applied unnecessarily to hole-punch in addition to truncation.
> > > > 
> > > > I am not sure much it should apply to hole-punch. Filesystem folks talk
> > > > about writing to a folio beyond round_up(i_size, PAGE_SIZE) being
> > > > problematic for correctness. I have no clue if the same applies to
> > > > writing to hole-punched parts of the folio.
> > > > 
> > > > Dave, any comments?
> > > > 
> > > > Hm. But if it is problematic it has be caught on fault. We don't do
> > > > this. It will be silently mapped.
> > > 
> > > There are strict rules about what happens beyond i_size, hence this
> > > patch.  But hole-punch has no persistent "i_size" to define it, and
> > > silently remapping in a fresh zeroed page is the correct behaviour.
> > 
> > I missed that we seems to be issuing vm_ops->page_mkwrite() on remaping
> > the page, so it is not completely silent for filesystem and can do its
> > thing to re-allocate metadata (or whatever) after hole-punch.
> > 
> > So, I see unmap on punch-hole being justified.
> 
> Most hole punching implementations in filesystems will take i_rwsem and
> mmap_invalidate lock, flush the range to disk and unmap the pagecache
> for all the fsblocks around that range, and only then update the file
> space mappings.  If the unmap fails because a PMD couldn't be split,
> then we'll just return that error to userspace and they can decide what
> to do when fallocate() fails.

Unmap does not fail and PMD can be split at any time. But split of large
folios can fail if there's an external pin on it.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov