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

[Sage Weil]

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) 

I fixed the bad GFP_KERNEL/uncheck kmalloc.  Which was just a struct 
pagevec, which we added a while ago to reduce stack usage.  Maybe the 
stack is a better place for it though?  (cifs puts it on the stack...)

> 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.

There are two other memory allocations during writeout: a vector of pages 
to be written, and the message we're sending to the OSD.  If I use a 
mempool for those to guarantee as least some writeout will occur, how do I 
safely defer when allocations do fail?  Will pdflush (or it's replacement) 
eventually come back and try ->writepages() again?

Thanks-
sage
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