From: Nadav Amit <nadav.amit@gmail.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
David Howells <dhowells@redhat.com>,
Al Viro <viro@zeniv.linux.org.uk>, Ted Ts'o <tytso@mit.edu>,
Dave Hansen <dave.hansen@linux.intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Linux-MM <linux-mm@kvack.org>,
Ext4 Developers List <linux-ext4@vger.kernel.org>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"?
Date: Tue, 22 Jun 2021 11:51:51 -0700 [thread overview]
Message-ID: <F78E1A78-DB7E-4F3A-8C7C-842AA757E4FE@gmail.com> (raw)
In-Reply-To: <YNItqqZA9Y1wOnZY@casper.infradead.org>
> On Jun 22, 2021, at 11:36 AM, Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Jun 22, 2021 at 11:28:30AM -0700, Linus Torvalds wrote:
>> On Tue, Jun 22, 2021 at 11:23 AM Matthew Wilcox <willy@infradead.org> wrote:
>>>
>>> It wouldn't be _that_ bad necessarily. filemap_fault:
>>
>> It's not actually the mm code that is the biggest problem. We
>> obviously already have readahead support.
>>
>> It's the *fault* side.
>>
>> In particular, since the fault would return without actually filling
>> in the page table entry (because the page isn't ready yet, and you
>> cannot expose it to other threads!), you also have to jump over the
>> instruction that caused this all.
>
> Oh, I was assuming that it'd be a function call like
> get_user_pages_fast(), not an instruction that was specially marked to
> be jumped over. Gag reflex diminishing now?
Just reminding the alternative (in the RFC that I mentioned before):
a vDSO exception table entry for a memory accessing function in the
vDSO. It then behaves as a sort of MADV_WILLNEED for the faulting
page if an exception is triggered. Unlike MADV_WILLNEED it maps the
page if no IO is needed. It can return through a register whether
the page was present or not.
I once implemented (another) alternative, in which the ELF had a section
with an exception-table (holding all the “Async-#PF” instructions),
which described where to skip to if a #PF occurs, but this solution
seemed too heavy-weight/intrusive.
next prev parent reply other threads:[~2021-06-22 18:51 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-22 15:20 Do we need to unrevert "fs: do not prefault sys_write() user buffer pages"? David Howells
2021-06-22 15:27 ` Al Viro
2021-06-22 15:36 ` Al Viro
2021-06-22 17:25 ` Matthew Wilcox
2021-06-22 17:39 ` Linus Torvalds
2021-06-22 17:55 ` David Howells
2021-06-22 18:04 ` Matthew Wilcox
2021-06-22 18:07 ` Linus Torvalds
2021-06-22 18:16 ` Nadav Amit
2021-06-22 18:23 ` Matthew Wilcox
2021-06-22 18:28 ` Linus Torvalds
2021-06-22 18:36 ` Matthew Wilcox
2021-06-22 18:51 ` Nadav Amit [this message]
2021-06-22 18:57 ` Linus Torvalds
2021-06-22 18:23 ` David Howells
2021-06-22 18:32 ` Linus Torvalds
2021-06-22 18:13 ` David Howells
2021-06-22 15:32 ` Linus Torvalds
2021-06-22 15:53 ` Linus Torvalds
2021-06-22 15:32 ` Matthew Wilcox
2021-06-22 16:27 ` David Howells
2021-06-22 21:55 ` David Laight
2021-06-22 22:04 ` Matthew Wilcox
2021-06-22 22:31 ` David Laight
2021-06-22 22:20 ` Dave Chinner
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