On Wed, 25 Aug 2010, Ted Ts'o wrote: > > 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. > > 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? -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
