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SubjectRe: [PATCH 3/3] autofs - fix AT_NO_AUTOMOUNT not being honored
On Thu, Nov 23 2017, Ian Kent wrote:

> On 23/11/17 08:47, NeilBrown wrote:
>> On Wed, Nov 22 2017, Ian Kent wrote:
>>> On 21/11/17 09:53, NeilBrown wrote:
>>>> On Wed, May 10 2017, Ian Kent wrote:
>>>>> The fstatat(2) and statx() calls can pass the flag AT_NO_AUTOMOUNT
>>>>> which is meant to clear the LOOKUP_AUTOMOUNT flag and prevent triggering
>>>>> of an automount by the call. But this flag is unconditionally cleared
>>>>> for all stat family system calls except statx().
>>>>> stat family system calls have always triggered mount requests for the
>>>>> negative dentry case in follow_automount() which is intended but prevents
>>>>> the fstatat(2) and statx() AT_NO_AUTOMOUNT case from being handled.
>>>>> In order to handle the AT_NO_AUTOMOUNT for both system calls the
>>>>> negative dentry case in follow_automount() needs to be changed to
>>>>> return ENOENT when the LOOKUP_AUTOMOUNT flag is clear (and the other
>>>>> required flags are clear).
>>>>> AFAICT this change doesn't have any noticable side effects and may,
>>>>> in some use cases (although I didn't see it in testing) prevent
>>>>> unnecessary callbacks to the automount daemon.
>>>> Actually, this patch does have a noticeable side effect.
>>> That's right.
>>>> Previously, if /home were an indirect mount point so that /home/neilb
>>>> would mount my home directory, "ls -l /home/neilb" would always work.
>>>> Now it doesn't.
>>> An this is correct too.
>>> The previous policy was that a ->lookup() would always cause a mount to
>>> occur. It's that policy that prevents the AT_NO_AUTOMOUNT flag from being
>>> able to work consistently.
>> Just to be clear, the previous policy was:
>> kernel - a lookup would cause a message to be sent to the automount daemon
>> daemon - the receipt of a message would cause a mount to occur.
>> Either part of this policy could be changed to allow AT_NO_AUTOMOUNT to
>> work reliably. You chose to change the first. At the time I thought
>> this was a good idea. I no longer think so.
>> Specifically, I think the second part of the policy should be revised a little.
>>> If you set the indirect mount "browse" option that will cause the mount
>>> point directories for each of the map keys to be pre-created. So a
>>> directory listing will show the mount points and an attempt to access
>>> anything within the mount point directory will cause it to be mounted.
>>> There is still the problem of not knowing map keys when the wildcard
>>> map entry is used.
>>> Still, stat family systems calls have always had similar problems that
>>> prevent consistent behavior.
>> Yes, I understand all this. stat family sys-call have some odd
>> behaviours at times like "stat(); open(); fstat()" will result in
>> different sets of status info. This is known.
>> The point is that these odd behaviours have changed in a noticeably way
>> (contrary to the change log comment) and it isn't clear these changes
>> are good.
>>>> This happens because "ls" calls 'lstat' on the path and when that fails
>>>> with "ENOENT", it doesn't bother trying to open.
>>> I know, I believe the ENOENT is appropriate because there is no mount
>>> and no directory at the time this happens.
>> Two distinct statements here. "no mount" and "no directory".
>> If AT_NO_AUTOMOUNT is given (or implicit) the "no mount" is significant
>> and shouldn't be changed. It isn't clear that "no directory" is
>> significant.
>> If you think of the list of names stored in the autofs filesystem as a
>> cache of recently used names from the map, then the directory *does*
>> exist (in the map), it is just that the (in-kernel) cache hasn't been
>> populated yet.
>> Should "AT_NO_AUTOMOUNT" prevent the cache from being populated?
>>>> An alternate approach to the problem that this patch addresses is to
>>>> *not* change follow_automount() but instead change the automount daemon
>>>> and possibly autofs.
>>> Perhaps, but the daemon probably doesn't have enough information to
>>> make decisions about this so there would need to be something coming
>>> from the kernel too.
>> I don't think so.
>> The daemon already has a promise that upcalls for a given name are
>> serialized, and it has the ability to test if a name is already in the
>> cache. This is enough.
>> I applied the following patch:
>> diff --git a/modules/mount_nfs.c b/modules/mount_nfs.c
>> index c558a7381054..7ddfe9c61038 100644
>> --- a/modules/mount_nfs.c
>> +++ b/modules/mount_nfs.c
>> @@ -269,6 +269,11 @@ dont_probe:
>> free_host_list(&hosts);
>> return 1;
>> }
>> + if (!status) {
>> + debug(ap->logopt, MODPREFIX "created dir, that'll do for now");
>> + free_host_list(&hosts);
>> + return 0;
>> + }
>> if (!status)
>> existed = 0;
>> to automount and now it behaves (for NFS mounts) how I would like (though
>> there is room for improvement).
>>>> When a notification of access for an indirect mount point is received,
>>>> it would firstly just create the directory - not triggering a mount.
>>>> If another notification is then received (after the directory has been
>>>> created), it then triggers the mount.
>>> Not sure, perhaps.
>>> But I don't think that will work, I've had many problems with this type
>>> behavior due to bugs and I think the the same sort of problems would be
>>> encountered.
>>> The indirect mount "browse" option which behaves very much like what your
>>> suggesting is the internal program default, and has been since the initial
>>> v5 release. Historically it is disabled on install to maintain backward
>>> compatibility with the original Linux autofs (which is also the reason for
>>> always triggering an automount on ->lookup()).
>>> Perhaps now is the time for that to change.
>> Enabling browse mode does resolve this problem when the map is
>> enumerable (as you say, wildcards can't be supported).
>> But browse mode isn't always wanted. If you have a very large map, then
>> caching all 10,000 entries in the kernel may be a pointless waste of
>> time and space.
> Indeed, that's the main use of nobrowse indirect maps.
> In fact another recent change where I moved the last_used field so it
> wouldn't be updated on every walk, to help with mounts never expiring,
> also needs at different approach.
> Doing that causes more aggressive expiring of automounts which increases
> umount to mount churn and leads to increased server load at sites with a
> very large number of clients.
>>>> I suspect this might need changes to autofs to avoid races when two
>>>> threads call lstat() on the same path at the same time. We would need
>>>> to ensure that automount didn't see this as two requests.... though
>>>> maybe it already has enough locking.
>>>> Does that seem reasonable? Should we revert this patch (as a
>>>> regression) and do something in automount instead?
>>> Can you check out the "browse" option behavior before we talk further
>>> about this please.
>> Done that.
>>> The problem in user space now is that there is no way to control
>>> whether an indirect mount that uses the "nobrowse" option is triggered
>>> or not (using fstatat() or statx()) regardless of the flags used and it's
>>> essential the AT_NO_AUTOMOUNT flag work as expected to have user space
>>> take more care in not triggering automounts.
>> I think that user-space has all the control that it needs.
>> There are two cases:
>> 1/ daemon receives "missing indirect" message and the directory
>> doesn't currently exist (mkdir() by the daemon succeeds).
>> This could be an AT_NO_AUTOMOUNT lookup, or it could be a full
>> open() or similar. In either case the directory gets created if
>> the map suggests that the name should exist. The daemon needs to
>> be careful not to block for long if it goes off-host to check for
>> validity of the name.
> Aren't you assuming the the daemon only receives these lookups
> on the last path component?

follow_managed() calls follow_automount() in a loop until it fails
(including -EISDIR which isn't exactly failure), or until no automount
is needed.
So where-ever the DCACHE_NEED_AUTOMOUNT is in the path, d_automount()
will be called if the dentry is negative, and unless it is the last
component of a NO_AUTOMOUNT lookup, it will be called if the dentry
isn't mounted-on. So it would now be called twice for indirect
mounts that aren't browseable - once to mkdir and once to mount. Might
there be a problem there?

> Intermediate path components that can trigger an automount must
> be treated as not having AT_NO_AUTOMOUNT in order for the walk to
> continue or fail at that point.
>> 2/ daemon received "missing indirect" message and the directory
>> *does* exist (mkdir() by daemon fails with -EEXIST).
>> This cannot be an AT_NO_AUTOMOUNT lookup, it must be a lookup
>> intended to trigger automounts. In this case, we trigger the
>> mount.
> An fstatat(<path>, AT_NO_AUTOMOUNT) on a browse indirect map entry
> means the directory will exist but user space has asked not to trigger
> the mount and return the stat info of the autofs dentry.
> Please don't get me wrong, I think the suggestion is good, I just
> don't think it's as simple to do as it appears.

Maybe I should write a more complete patch for you to experiment with.


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