linux-kernel - Re: [PATCH v2 0/9] support large folio swap-out and swap-in for shmem

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Date: Thu, 20 Jun 2024 08:52:57 +0200
From: David Hildenbrand <david@...hat.com>
To: Hugh Dickins <hughd@...gle.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Barry Song <baohua@...nel.org>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, willy@...radead.org,
 wangkefeng.wang@...wei.com, chrisl@...nel.org, ying.huang@...el.com,
 21cnbao@...il.com, ryan.roberts@....com, shy828301@...il.com,
 ziy@...dia.com, ioworker0@...il.com, da.gomez@...sung.com,
 p.raghav@...sung.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/9] support large folio swap-out and swap-in for shmem

> 
> (I do have doubts about Barry's: the "_new" in folio_add_new_anon_rmap()
> was all about optimizing a known-exclusive case, so it surprises me
> to see it being extended to non-exclusive; and I worry over how its
> atomic_set(&page->_mapcount, 0)s can be safe when non-exclusive (but
> I've never caught up with David's exclusive changes, I'm out of date).

We discussed that a while ago: if we wouldn't be holding the folio lock 
in the "folio == swapcache" at that point (which we do for both 
do_swap_page() and unuse_pte()) things would already be pretty broken.

That's I added a while ago:

if (unlikely(!folio_test_anon(folio))) {
	VM_WARN_ON_FOLIO(!folio_test_locked(folio), folio);
	/*
	 * For a PTE-mapped large folio, we only know that the single
	 * PTE is exclusive. Further, __folio_set_anon() might not get
	 * folio->index right when not given the address of the head
	 * page.
	 */
	...

We should probably move that VM_WARN_ON_FOLIO() to 
folio_add_new_anon_rmap() and document that it's required in the 
non-exclusive case.

> 
> But even if those are wrong, I'd expect them to tend towards a mapped
> page becoming unreclaimable, then "Bad page map" when munmapped,
> not to any of the double-free symptoms I've actually seen.)

What's the first known-good commit?

-- 
Cheers,

David / dhildenb