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From: Matthew Wilcox <willy@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Will Deacon <will@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Jan Kara <jack@suse.cz>, Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vinayak Menon <vinmenon@codeaurora.org>,
	Android Kernel Team <kernel-team@android.com>
Subject: Re: [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting
Date: Mon, 14 Dec 2020 18:56:10 +0000	[thread overview]
Message-ID: <20201214185610.GO2443@casper.infradead.org> (raw)
In-Reply-To: <CAHk-=wi80Qp6nZC0yyewhnqvrmPx2h_yWvfq4A25ONb7z9BywQ@mail.gmail.com>

On Mon, Dec 14, 2020 at 09:54:06AM -0800, Linus Torvalds wrote:
> > I expected to hate it more, but it looks reasonable. Opencoded
> > xas_for_each() smells bad, but...
> 
> I think the open-coded xas_for_each() per se isn't a problem, but I
> agree that the startup condition is a bit ugly. And I'm actually
> personally more confused by why xas_retry() is needed here, bit not in
> many other places. That is perhaps more obvious now that it shows up
> twice.
> 
> Adding Willy to the cc in case he has comments on that, and can
> explain it to me in small words.
> 
> [ https://lore.kernel.org/lkml/20201214160724.ewhjqoi32chheone@box/
> for context ]

The xas_retry() is something I now regret, but haven't got annoyed enough
by it yet to fix (also, other projects).  It originated in the radix
tree where we would get a radix_tree_node and then iterate over it in
header macros.  If we're holding the rcu_read_lock() and somebody else
deletes an entry leaving the entry at index 0 as the only index in the
tree, we tell the RCU readers to rewalk the tree from the top by putting
a retry entry in place of the real entry.

It's not entirely clear to me now why we did that.  Just leave the entry
alone and the RCU-walkers will see it, then the rest of the node is empty.

As to why we need to do this in some places and not others; you can
only see a retry entry if you're only protected by the RCU lock.  If
you're protected by the spinlock, you can't see any nodes which
contain retry entries.

But I now think we should just get rid of retry entries.  Maybe I'm
missing a good reason to keep them.

  reply	other threads:[~2020-12-14 18:57 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-09 16:39 [PATCH 0/2] Create 'old' ptes for faultaround mappings on arm64 with hardware access flag Will Deacon
2020-12-09 16:39 ` [PATCH 1/2] mm: Allow architectures to request 'old' entries when prefaulting Will Deacon
2020-12-09 17:58   ` Linus Torvalds
2020-12-09 18:40     ` Will Deacon
2020-12-09 19:04       ` Linus Torvalds
2020-12-09 20:32         ` Matthew Wilcox
2020-12-09 21:04           ` Linus Torvalds
2020-12-10 15:08         ` Kirill A. Shutemov
2020-12-10 17:23           ` Linus Torvalds
2020-12-14 16:07             ` Kirill A. Shutemov
2020-12-14 17:54               ` Linus Torvalds
2020-12-14 18:56                 ` Matthew Wilcox [this message]
2020-12-16 17:07                 ` Kirill A. Shutemov
2020-12-16 18:41                   ` Linus Torvalds
2020-12-17 10:54                     ` Kirill A. Shutemov
2020-12-17 18:22                       ` Linus Torvalds
2020-12-18 11:04                         ` Kirill A. Shutemov
2020-12-18 18:56                           ` Linus Torvalds
2020-12-19 12:41                             ` Kirill A. Shutemov
2020-12-19 20:08                               ` Linus Torvalds
2020-12-19 20:34                               ` Linus Torvalds
2020-12-22 10:00                                 ` Kirill A. Shutemov
2020-12-24  4:04                                   ` Hugh Dickins
2020-12-25 11:31                                     ` Kirill A. Shutemov
2020-12-26 17:57                                       ` Linus Torvalds
2020-12-26 20:43                                         ` Kirill A. Shutemov
2020-12-26 21:03                                           ` Hugh Dickins
2020-12-26 21:16                                             ` Linus Torvalds
2020-12-26 22:40                                               ` Kirill A. Shutemov
2020-12-27  0:45                                                 ` Hugh Dickins
2020-12-27  2:38                                                   ` Hugh Dickins
2020-12-27 19:38                                                     ` Linus Torvalds
2020-12-27 20:32                                                       ` Damian Tometzki
2020-12-27 22:35                                                         ` Hugh Dickins
2020-12-27 23:12                                                           ` Linus Torvalds
2020-12-27 23:40                                                             ` Linus Torvalds
2020-12-27 23:55                                                               ` Kirill A. Shutemov
2020-12-27 23:48                                                       ` Kirill A. Shutemov
2020-12-28  1:54                                                         ` Linus Torvalds
2020-12-28  6:43                                                           ` Hugh Dickins
2020-12-28 12:53                                                             ` Kirill A. Shutemov
2020-12-28 18:47                                                               ` Linus Torvalds
2020-12-28 21:58                                                                 ` Linus Torvalds
2020-12-29 13:28                                                                   ` Kirill A. Shutemov
2020-12-29 15:19                                                                     ` Matthew Wilcox
2020-12-29 20:52                                                                     ` Linus Torvalds
2020-12-28 22:05                                                                 ` Kirill A. Shutemov
2020-12-28 22:12                                                                   ` Kirill A. Shutemov
2020-12-29  4:35                                                                     ` Hugh Dickins
2020-12-28 23:28                                                                   ` Linus Torvalds
2020-12-26 21:07                                           ` Linus Torvalds
2020-12-26 21:41                                           ` Matthew Wilcox
2020-12-09 16:39 ` [PATCH 2/2] arm64: mm: Implement arch_wants_old_faultaround_pte() Will Deacon
2020-12-09 18:35   ` Catalin Marinas
2020-12-09 18:46     ` Will Deacon

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