Re: [PATCH 08/19] ceph: address space operations

[Sage Weil]

A related question I had on writepages failures: what is the 'right' thing 
to do if we get a server error on writeback?  If we believe it may be 
transient (say, ENOSPC), should we redirty pages and hope for better luck 
next time?

What if we decide it's a fatal error?

sage


On Thu, 23 Jul 2009, Andi Kleen wrote:

> Sage Weil <sage@xxxxxxxxxxxx> writes:
> 
> > The ceph address space methods are concerned primarily with managing
> > the dirty page accounting in the inode, which (among other things)
> > must keep track of which snapshot context each page was dirtied in,
> > and ensure that dirty data is written out to the OSDs in snapshort
> > order.
> >
> > A writepage() on a page that is not currently writeable due to
> > snapshot writeback ordering constraints is ignored (it was presumably
> > called from kswapd).
> 
> Not a detailed review. You would need to get one from someone who
> knows the VFS interfaces very well (unfortunately those people are hard
> to find). I just read through it.
> 
> One thing I noticed is that you seem to do a lot of memory allocation
> in the write out paths (some of it even GFP_KERNEL, not GFP_NOFS) 
> 
> The traditional wisdom is that you should not allocate memory in block
> writeout, because that can deadlock. The worst case is swapfile
> on it, but it can happen with mmap too (e.g. one process using
> most memory with a file mmap from your fs)  GFP_KERNEL can also recurse,
> which can cause other problems in your fs.
> 
> There were some changes to make this problem less severe (e.g. better
> dirty pages accounting), but I don't think anyone has really declared
> it solved yet. The standard workaround for this is to use mempools 
> for anything allocated in the writeout path, then you are at least
> guaranteed to make forward progress.
> 
> You also had at least one unchecked kmalloc I think.
> 
> -Andi
> 
> -- 
> ak@xxxxxxxxxxxxxxx -- Speaking for myself only.
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