linux-kernel - Re: [PATCH v2 1/2] mm/memfd: refactor memfd_tag_pins() and memfd_wait_for

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Date: Tue, 27 Feb 2024 15:27:16 +0000
From: Matthew Wilcox <willy@...radead.org>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 1/2] mm/memfd: refactor memfd_tag_pins() and
 memfd_wait_for_pins()

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@...radead.org>