[Devel] Re: [PATCH 2.6.19-rc3] VFS: per-sb dentry lru list

[Vasily Averin]

Hello Neil,

Neil Brown wrote:
> On Tuesday October 31, dhowells at redhat.com wrote:
>> Neil Brown <neilb at suse.de> wrote:
>>
>>> When we unmount a filesystem we need to release all dentries.
>>> We currently
>>>   - move a collection of dentries to the end of the dentry_unused list
>>>   - call prune_dcache to prune that number of dentries.
>> This is not true anymore.
> 
> True.  That should read:
> 
>  When we remount a filesystem or invalidate a block device which has a
>  mounted filesystem we call shrink dcache_sb which currently:
>      - moves a collection of dentries to the end of the dentry_unused list
>      - calls prune_dcache to prune that number of dentries.
> 
> but the patch is still valid.
> 
> Any objections to it going in to -mm  and maybe .20 ??

Currently we have 3 type of functions that works with dentry_unused list:

1) prune_dcache(NULL) -- called from shrink_dcache_memory, frees the memory and
requires global LRU. works well in current implementation.
2) prune_dcache(sb)  -- called from shrink_dcache_parent(), frees subtree, LRU
is not need here.  Current implementation uses global LRU for these purposes, it
is ineffective, and patch from Neil Brown fixes this issue.
3) shrink_dcache_sb() -- called when we need to free the unused dentries for
given super block. Current implementation is not effective too, and per-sb LRU
would be the best solution here. On the other hand patch from Neil Brown is much
better than current implementation.

In general I think that we should approve Neil Brown's patch. We (I and Kirill
Korotaev) are ready to acknowledge it when the following remarks fill be fixed:

- it seems for me list_splice() is not required inside prune_dcache(),
- DCACHE_REFERENCED dentries should not be removed from private list to
dentry_unused list, this flag should be ignored if the private list is used,
- count argument should be ignored in this case too, we want to free all the
dentries in private list,
- when we shrink the whole super block we should free per-sb anonymous dentries
too (please see Kirill Korotaev's letter)

Then I'm going to prepare new patch that will enhance the shrink_dcache_sb()
performance:
- we can add new list head into struct superblock and use it in
shrink_dcache_sb() instead of temporal private list. We will check is it empty
in dput() and add the new unused dentries to per-sb list instead of
dentry_unused list.

thank you,
	Vasily Averin

SWsoft Virtuozzo/OpenVZ Linux kernel team