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Message-ID: <02e4e497-d70f-4f4b-8b2c-55fe3a0b726e@redhat.com>
Date: Mon, 26 Feb 2024 17:56:09 +0100
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>
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 26.02.24 17:07, Matthew Wilcox wrote:
> On Mon, Feb 26, 2024 at 03:13:23PM +0100, David Hildenbrand wrote:
>> + xas_for_each(xas, folio, ULONG_MAX) {
>> + if (!xa_is_value(folio) && memfd_folio_has_extra_refs(folio))
>> xas_set_mark(xas, MEMFD_TAG_PINNED);
>
> ... we decline to tag value entries here ...
>
>> @@ -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?
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.
Thanks!
--
Cheers,
David / dhildenb
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