On Mon, Feb 26, 2024 at 05:56:09PM +0100, David Hildenbrand wrote:
> > > @@ -95,20 +90,15 @@ static int memfd_wait_for_pins(struct address_space *mapping)
> > > xas_set(&xas, 0);
> > > xas_lock_irq(&xas);
> > > - xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
> > > + xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) {
> > > bool clear = true;
> > > - cache_count = 1;
> > > - if (!xa_is_value(page) &&
> > > - PageTransHuge(page) && !PageHuge(page))
> > > - cache_count = HPAGE_PMD_NR;
> > > -
> > > - if (!xa_is_value(page) && cache_count !=
> > > - page_count(page) - total_mapcount(page)) {
> > > + if (!xa_is_value(folio) &&
> > > + memfd_folio_has_extra_refs(folio)) {
> >
> > ... so we don't need to test it here because we'll never see any value
> > entries. No?
>
> I was not able to convince myself that swapout code would clear the mark
> when replacing the entry.
>
> shmem_writepage()->shmem_delete_from_page_cache()->shmem_replace_entry()
>
> will perform a xas_store() with swp_to_radix_entry(swap) under
> xa_lock_irq().
>
> Reading the doc, and staring at the code for a bit too long, I think
> xas_store() would only clear tags when deleting an entry (passing NULL).
>
> But maybe xas_store() will always clear tags?
No, xas_store() will leave the tag alone ... this is the right thing to
do for the pagecache because we always clear the tags before removing
a folio from the cache.
> In memfd code, I think we could see swapout between memfd_tag_pins() and the
> check for tags, where we drop the xa_lock. Unless some other lock (inode
> lock?) protects us.
... and if it does happen, we see the value entry tagged and clear the
tag on it. OK.
Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
