Re: [patch 1/5] mm: add nofail variants of kmalloc kcalloc and kzalloc

On Wed, 25 Aug 2010, Ted Ts'o wrote:

quoted

We certainly hope that nobody will reimplement the same function without 
the __deprecated warning, especially for order < PAGE_ALLOC_COSTLY_ORDER 
where there's no looping at a higher level.  So perhaps the best 
alternative is to implement the same _nofail() functions but do a 
WARN_ON(get_order(size) > PAGE_ALLOC_COSTLY_ORDER) instead?

Yeah, that sounds better.

Ok, and we'll make it a WARN_ON_ONCE() to be nice to the kernel log.  
Although the current patchset does this with WARN_ON_ONCE(1, ...) instead, 
this serves to ensure that we aren't dependent on the page allocator's 
implementation to always loop for order < PAGE_ALLOC_COSTLY_ORDER in which 
case the loop in the _nofail() functions would actually do something.

quoted

I think it's really sad that the caller can't know what the upper bounds 
of its memory requirement are ahead of time or at least be able to 
implement a memory freeing function when kmalloc() returns NULL.

Oh, we can determine an upper bound.  You might just not like it.
Actually ext3/ext4 shouldn't be as bad as XFS, which Dave estimated to
be around 400k for a transaction.  My guess is that the worst case for
ext3/ext4 is probably around 256k or so; like XFS, most of the time,
it would be a lot less.  (At least, if data != journalled; if we are
doing data journalling and every single data block begins with
0xc03b3998U, we'll need to allocate a 4k page for every single data
block written.)  We could dynamically calculate an upper bound if we
had to.  Of course, if ext3/ext4 is attached to a network block
device, then it could get a lot worse than 256k, of course.

On my 8GB machine, /proc/zoneinfo says the min watermark for ZONE_NORMAL 
is 5086 pages, or ~20MB.  GFP_ATOMIC would allow access to ~12MB of that, 
so perhaps we should consider this is an acceptable abuse of GFP_ATOMIC as 
a fallback behavior when GFP_NOFS or GFP_NOIO fails?