RE: [PATCH v3 0/5] fs/dcache: Limit # of negative dentries

[Wangkai (Kevin,C)]

> -----Original Message-----
> From: Waiman Long [mailto:longman@xxxxxxxxxx]
> Sent: Thursday, August 17, 2017 9:04 PM
> To: Wangkai (Kevin,C); Alexander Viro; Jonathan Corbet
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-doc@xxxxxxxxxxxxxxx;
> linux-fsdevel@xxxxxxxxxxxxxxx; Paul E. McKenney; Andrew Morton; Ingo Molnar;
> Miklos Szeredi; Matthew Wilcox; Larry Woodman; James Bottomley
> Subject: Re: [PATCH v3 0/5] fs/dcache: Limit # of negative dentries
> 
> On 08/17/2017 12:00 AM, Wangkai (Kevin,C) wrote:
> >
> >>>
> >>> Hi Longman,
> >>> I am a fresher of fsdevel, about 2 weeks before, I have joined this
> >>> mail list, recently I have met the same problem of negative
> >>> dentries, in my opinion, the dentries should be remove together with
> >>> the files or
> >> directories, I don't know you have submit this patch, I have another
> >> patch about this:
> >>> http://marc.info/?l=linux-fsdevel&m=150209902215266&w=2
> >>>
> >>> maybe this is a foo idea...
> >>>
> >>> regards
> >>> Kevin
> >> If you look at the code, the front dentries of the LRU list are
> >> removed when there are too many negative dentries. That includes
> >> positive dentries as well as it is not practical to just remove the negative
> dentries.
> >>
> >> I have looked at your patch. The dentry of a removed file becomes a
> >> negative dentry. The kernel can keep track of those negative entries
> >> and there is no need to add an additional flag for that.
> >>
> >> Cheers,
> >> Longman
> > One comment about your patch:
> > In the patch 1/5 function dentry_kill first get dentry->d_flags, after
> > lock parent and Compare d_flags again, is this needed? The d_flags was
> changed under lock.
> 
> Yes, it is necessary. We are talking about an SMP system with multiple threads
> running concurrently. If you look at the lock parent code, it may release the
> current dentry lock before taking the parent's and then the dentry lock again.
> As soon as the lock is released, anything can happen to the dentry including
> changes in d_flags.

Yes, I am not check the lock parent code, it is necessary.

> > In my patch the DCACHE_FILE_REMOVED flag was to distinguish the
> > removed file and The closed file, I found there was no difference of a
> > dentry between the removed file and the closed File, they all on the lru list.
> 
> There is a difference between removed file and closed file. The type field of
> d_flags will be empty for a removed file which indicate a negative dentry.
> Anything else is a positive dentry. Look at the inline function d_is_negative()
> [d_is_miss()] and you will see how it is done.

After the file was removed, the dentry flag was not MISS, the flag was:
DCACHE_REFERENCED | DCACHE_RCUACCESS | DCACHE_LRU_LIST | DCACHE_REGULAR_TYPE
So, the dentry never be freed, until the kernel reclaim the slab memory.

Regards,
Kevin