Re: [PATCHv2 1/2] mm/memory: Do not populate page table entries beyond i_size

[Hugh Dickins <hughd@google.com>]

On Wed, 29 Oct 2025, Kiryl Shutsemau wrote:
> On Wed, Oct 29, 2025 at 01:31:45AM -0700, Hugh Dickins wrote:
> > On Mon, 27 Oct 2025, David Hildenbrand wrote:
> > ...
> > > 
> > > Just so we are on the same page: this is not about which folio sizes we
> > > allocate (like what Baolin fixed) but what/how much to map.
> > > 
> > > I guess this patch here would imply the following changes
> > > 
> > > 1) A file with a size that is not PMD aligned will have the last (unaligned
> > > part) not mapped by PMDs.
> > > 
> > > 2) Once growing a file, the previously-last-part would not be mapped by PMDs.
> > 
> > Yes, the v2 patch was so, and the v3 patch fixes it.
> > 
> > khugepaged might have fixed it up later on, I suppose.
> > 
> > Hmm, does hpage_collapse_scan_file() or collapse_pte_mapped_thp()
> > want a modification, to prevent reinserting a PMD after a failed
> > non-shmem truncation folio_split?  And collapse_file() after a
> > successful non-shmem truncation folio_split?
> 
> I operated from an assumption that file collapse is still lazy as I
> wrote it back it the days and doesn't install PMDs. It *seems* to be
> true for khugepaged, but not MADV_COLLAPSE.
> 
> Hm...
> 
> > Conversely, shouldn't MADV_COLLAPSE be happy to give you a PMD
> > if the map size permits, even when spanning EOF?
> 
> Filesystem folks say allowing the folio to be mapped beyond
> round_up(i_size, PAGE_SIZE) is a correctness issue, not only POSIX
> violation.
> 
> I consider dropping 'install_pmd' from collapse_pte_mapped_thp() so the
> fault path is source of truth of whether PMD can be installed or not.

(Didn't you yourself just recently enhance that?)

> 
> Objections?

Yes, I would probably object (or perhaps want to allow until EOF);
but now it looks to me like we can agree no change is needed there.

I was mistaken in raising those khugepaged/MADV_COLLAPSE doubts,
because file_thp_enabled(vma) is checked in the !shmem !anonymous
!dax case, and file_thp_enabled(vma) still limits to
CONFIG_READ_ONLY_THP_FOR_FS=y, refusing to allow collapse if anyone
has the file open for writing (and you cannot truncate or hole-punch
without write permission); and pagecache is invalidated afterwards
if there are any THPs when reopened for writing (presumably for
page_mkwrite()-ish consistency reasons, which you interestingly
pointed to in another mail where I had worried about ENOSPC after
split failure).

But shmem is simple, does not use page_mkwrite(), and is fine to
continue with install_pmd here, just as it's fine to continue
with huge page spanning EOF as you're now allowing in v3.

But please double check my conclusion there, it's so easy to
get lost in the maze of hugepage permissions and prohibitions.

Hugh