From: Ian Kent <ikent@redhat.com>
To: mtk.manpages@gmail.com, NeilBrown <neilb@suse.com>
Cc: Jeff Layton <jlayton@redhat.com>,
Trond Myklebust <trondmy@primarydata.com>,
"viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"mkoutny@suse.com" <mkoutny@suse.com>,
"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
David Howells <dhowells@redhat.com>,
"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: Do we really need d_weak_revalidate???
Date: Fri, 25 Aug 2017 08:05:09 +0800 [thread overview]
Message-ID: <9c4b3c7a-6b05-eb8e-72b9-d95a1cdc62ed@redhat.com> (raw)
In-Reply-To: <CAKgNAki1SYjr7-6g-i3shGjQRjwy81gS0QisfZRy-x-bHFtXmA@mail.gmail.com>
On 24/08/17 19:03, Michael Kerrisk (man-pages) wrote:
> Hi Neil,
>
> On 24 August 2017 at 06:07, NeilBrown <neilb@suse.com> wrote:
>> On Wed, Aug 23 2017, Ian Kent wrote:
>>
>>>
>>> That inconsistency has bothered me for quite a while now.
>>>
>>> It was carried over from the autofs module behavior when automounting
>>> support was added to the VFS. What's worse is it prevents the use of
>>> the AT_NO_AUTOMOUNT flag from working properly with fstatat(2) and with
>>> statx().
>>>
>>> There is some risk in changing that so it does work but it really does
>>> need to work to enable userspace to not trigger an automount by using
>>> this flag.
>>>
>>> So that's (hopefully) going to change soonish, see:
>>> http://ozlabs.org/~akpm/mmotm/broken-out/autofs-fix-at_no_automount-not-being-honored.patch
>>>
>>> The result should be that stat family calls don't trigger automounts except
>>> for fstatat(2) and statx() which will require the AT_NO_AUTOMOUNT flag.
>>>
>>
>> oooh, yes. That's much better - thanks.
>>
>> We should make sure that change gets into the man pages...
>>
>> First however, we should probably correct the man page!
>> stat.2 says:
>>
>>
>> NOTES
>> On Linux, lstat() will generally not trigger automounter
>> action, whereas stat() will (but see the description of
>> fstatat() AT_NO_AUTOMOUNT fag, above).
>>
>> which is wrong: lstat and stat treat automounts the same.
>> @Michael: do you recall why you inserted that text? The commit message
>> in commit 1ef5b2805471 ("stat.2: Cosmetic reworking of timestamp
>> discussion in NOTES") is not very helpful.
>
> That commit really was just cosmetic changes. The change that
> introduced the text was 82d2be3d9d66b7, based on a note from Peter
> Anvin:
Indeed, that was correct for autofs v3 but we're at autofs v5 now and
a lot has changed over time (the commit is from 2008).
All I can do is apologize for not also checking the man pages and trying
to keep them up to date.
Let's just work on making them accurate now.
>
> [[
> > > Additionally, you may want to make a note in the stat/lstat man page tha
> t on
> > > Linux, lstat(2) will generally not trigger automounter action, whereas
> > > stat(2) will.
> >
> > I don't understand this last piece. Can you say some more. (I'm not
> > familiar with automounter details.)
>
> An automounter (either an explicit one, like autofs, or an implicit
> one, such as are used by AFS or NFSv4) is something that triggers
> a mount when something is touched.
>
> However, it's undesirable to automount, say, everyone's home
> directory just because someone opened up /home in their GUI
> browser or typed "ls -l /home". The early automounters simply
> didn't list the contents until you accessed it by name;
> this is still the case when you can't enumerate a mapping
> (say, all DNS names under /net). However, this is extremely
> inconvenient, too.
>
> The solution we ended up settling on is to create something
> that looks like a directory (i.e. reports S_IFDIR in stat()),
> but behaves somewhat like a symlink. In particular, when it is
> accessed in a way where a symlink would be dereferenced,
> the automount triggers and the directory is mounted. However,
> system calls which do *not* cause a symlink to be dereferenced,
> like lstat(), also do not cause the automounter to trigger.
> This means that "ls -l", or a GUI file browser, can see a list
> of directories without causing each one of them to be automounted.
>
> -hpa
> ]]
>
> Cheers,
>
> Michael
>
>> I propose correcting to
>>
>> NOTES:
>> On Linux, lstat() nor stat() act as though AT_NO_AUTOMOUNT was set
>> and will not trigger automounter action for direct automount
>> points, though they may (prior to 4.14) for indirect automount
>> points.
>>
>>
>> The more precise details, that automount action for indirect automount
>> points is not triggered when the 'browse' option is used, is probably
>> not necessary.
>>
>> Ian: if you agree with that text, and Michael doesn't provide alternate
>> evidence, I'll submit a formal patch for the man page.... or should we
>> just wait until the patch actually lands?
>>
>> Thanks,
>> NeilBrown
>>
>
>
>
next prev parent reply other threads:[~2017-08-25 0:05 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-11 4:31 Do we really need d_weak_revalidate??? NeilBrown
2017-08-11 5:55 ` Trond Myklebust
2017-08-11 11:01 ` Jeff Layton
2017-08-13 23:36 ` NeilBrown
2017-08-14 10:10 ` Jeff Layton
2017-08-16 2:43 ` NeilBrown
2017-08-16 11:34 ` Jeff Layton
2017-08-16 23:47 ` NeilBrown
2017-08-17 2:20 ` Ian Kent
2017-08-18 5:24 ` NeilBrown
2017-08-18 6:47 ` Ian Kent
2017-08-18 6:55 ` Ian Kent
2017-08-21 6:23 ` NeilBrown
2017-08-21 6:32 ` Ian Kent
2017-08-21 7:46 ` NeilBrown
2017-08-23 1:06 ` NeilBrown
2017-08-23 2:32 ` Ian Kent
2017-08-23 2:40 ` Ian Kent
2017-08-23 2:54 ` Ian Kent
2017-08-23 7:51 ` Ian Kent
2017-08-24 3:21 ` NeilBrown
2017-08-24 4:35 ` Ian Kent
2017-08-24 4:07 ` NeilBrown
2017-08-24 4:47 ` Ian Kent
2017-08-24 4:58 ` Ian Kent
2017-08-24 11:03 ` Michael Kerrisk (man-pages)
2017-08-25 0:05 ` Ian Kent [this message]
2017-08-25 5:32 ` [PATCH manpages] stat.2: correct AT_NO_AUTOMOUNT text and general revisions NeilBrown
2017-09-14 13:38 ` Michael Kerrisk (man-pages)
2017-09-14 22:25 ` NeilBrown
2017-09-16 13:11 ` Michael Kerrisk (man-pages)
2017-09-08 15:15 ` Do we really need d_weak_revalidate??? David Howells
2017-08-13 23:29 ` NeilBrown
2017-08-24 6:34 ` NeilBrown
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=9c4b3c7a-6b05-eb8e-72b9-d95a1cdc62ed@redhat.com \
--to=ikent@redhat.com \
--cc=dhowells@redhat.com \
--cc=hpa@zytor.com \
--cc=jlayton@redhat.com \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-nfs@vger.kernel.org \
--cc=mkoutny@suse.com \
--cc=mtk.manpages@gmail.com \
--cc=neilb@suse.com \
--cc=trondmy@primarydata.com \
--cc=viro@zeniv.linux.org.uk \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).