Re: [PATCH] ext4: delayed allocation i_blocks fix for stat(2)

Mingming Cao
2008-07-08T22:55:21Z

在 2008-07-08二的 22:02 +0200，Jan Kara写道：
> > Eric Sandeen wrote:
> > >Mingming Cao wrote:
> > >
> > >>ext4: delayed allocation i_blocks fix for stat(2)
> > >>
> > >>From: Mingming Cao <cmm@us.ibm.com>
> > >>
> > >>Right now i_blocks is not getting updated until the disks are actually
> > >>allocaed on disk. This means with delayed allocation, right after files
> > >>are copied, "ls -sF" shoes the file as taking 0 blocks on disk. "du"
> > >>also shows the files taking zero space, which is highly confusing to the
> > >>user.
> > >>
> > >>Since current delayed allocation already keep track of per-inode total
> > >>number
> > >>of blocks that are subject to delayed allocation, this patch fix this by
> > >>using
> > >>that to adjust the value returned by stat(2). When real block allocation
> > >>is done, the i_blocks will get updated. Since the reserved blocks for
> > >>delayed
> > >>allocation will be decreased, this will be keep value returned by stat(2)
> > >>consistent.
> > >>
> > >>Signed-off-by: Mingming Cao <cmm@us.ibm.com>
> > >>
> > >
> > >Thanks Mingming, looks like just the right approach.
> > >
> > >Something about the spinlock for every stat seems heavy-handed to me but
> > >I'll have to give that more thought. :)
> > >
> > >
> >
> > Since i_reserved_blocks is an unsigned long, it should be possible
> > to atomically fetch it on all of the supported architectures,
> > without the use of the spinlock. It seems to me that this spinlock
> > is not required here.
> Well, it's certainly not nice to rely on this. The clean solution
> would be to convert i_reserved_blocks to atomic_t or atomic64_t on archs
> that have it...
>

I was thinking about the same thing when this lock was initially
introduced ... but this lock is protecting three counters, and these
counters are updated/reset in a couple of places, and being used to
calcuate how much per-fs free blocks counter need to be accounted. It's
doable, just need a careful look if we make them all atomic.

Mingming

> Honza

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