Re: [PATCH] Busy inodes after unmount, be more verbose in generic_shutdown_super

[Kirill Korotaev <dev@sw.ru>]

>>No, it looks as it is not :(
>>Have you noticed my comment about "count" argument to prune_dcache()?
>>For example, prune_dcache() is called from shrink_dcache_parent() which 
>>is called in many places and not all of them have PF_MEMALLOC or 
>>s_umount semaphore for write. But prune_dcache() doesn't care for super 
>>blocks etc. It simply shrinks N dentries which are found _first_.
>>
>>So the condition:
>>+		if ((current->flags & PF_MEMALLOC) &&
>>+			!(ret = down_read_trylock(&s->s_umount))) {
>>is not always true when the race occurs, as PF_MEMALLOC is not always set.
> 
> 
> I understand your comment about shrink_dcache_parent() being called
> from several places. prune_one_dentry() would eventually dput the parent,
> but unmount would go ahead and unmount the filesystem before the
> dput of the parent could happen.
exactly.

> Given that background, I thought our main concern was with respect to
> unmount. The race was between shrink_dcache_parent() (called from unmount)
> and shrink_dcache_memory() (called from the allocator), hence the fix
> for the race condition.
Partial fix doesn't make much sense from my point of view.

> I just noticied that 2.6.16-rc* now seems to have drop_slab() where
> PF_MEMALLOC is not set. So, we can still race with my fix if there
> if /proc/sys/vm/drop_caches is written to and unmount is done in parallel.
> 
> A simple hack would be to set PF_MEMALLOC in drop_slab(), but I do not
> think it is a good idea.
Yeah, playing with PF_MEMALLOC can be not so good idea :/
And as it doesn't help in other cases it looks unpromising...

>>>Have you had any other feedback on this?
>>here it is :)
> Thanks for your detailed feedback
Sorry, that I did it too late :/

Thanks,
Kirill