linux-kernel - Re: [PATCH v5 4/9] mm: filemap: use xa_get

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Message-ID: <Zsu-OCxgB9OAK050@casper.infradead.org>
Date: Mon, 26 Aug 2024 00:28:56 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Hugh Dickins <hughd@...gle.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>, akpm@...ux-foundation.org,
	david@...hat.com, 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 v5 4/9] mm: filemap: use xa_get_order() to get the swap
 entry order

On Sun, Aug 25, 2024 at 02:55:41PM -0700, Hugh Dickins wrote:
> The second issue is that swap is more slippery to work with than
> folios or pages: in the folio_nr_pages() case, that number is stable
> because we hold a refcount (which stops a THP from being split), and
> later we'll be taking folio lock too.  None of that in the swap case,
> so (depending on how a large entry gets split) the xa_get_order() result
> is not reliable. Likewise for other uses of xa_get_order() in this series.
> 
> There needs to be some kind of locking or retry to make the order usable,
> and to avoid shmem_free_swap() occasionally freeing more than it ought.
> I'll give it thought after.

My original thought was that we'd take a bit from the swap entry in
order to indicate the order of the entry.  I was surprised to see the
xa_get_order() implementation, but didn't remember why it wouldn't work.
Sorry.

Anyway, that's how I think it should be fixed.  Is that enough?  Holding
a reference on the folio prevents truncation, splitting, and so on.
There's no reference to be held on a swap entry, so could we have some
moderately implausible series of operations while holding only the RCU
read lock that would cause us to go wrong?