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Date: Tue, 27 Jun 2006 10:22:40 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Nathan Scott <nathans@sgi.com>
Cc: Christoph Hellwig <hch@infradead.org>,
       Steven Whitehouse <swhiteho@redhat.com>,
       Linus Torvalds <torvalds@osdl.org>,
       David Teigland <teigland@redhat.com>,
       Patrick Caulfield <pcaulfie@redhat.com>,
       Kevin Anderson <kanderso@redhat.com>, Andrew Morton <akpm@osdl.org>,
       linux-kernel@vger.kernel.org
Subject: Re: GFS2 and DLM
Message-ID: <20060627082240.GA672@elte.hu>
References: <1150805833.3856.1356.camel@quoit.chygwyn.com> <20060623144928.GA32694@infradead.org> <20060626200300.GA15424@elte.hu> <20060627063339.GA27938@elte.hu> <20060627181632.A1297906@wobbly.melbourne.sgi.com>
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* Nathan Scott <nathans@sgi.com> wrote:

> >           */
> >          buf = (xfs_caddr_t) kmem_zalloc(NBPP, KM_SLEEP);
> >  [...]
> > 
> > where kmem_zalloc() may fail!!!
> 
> Not with the flags it was given.

yeah, you are right - sorry about that.

XFS instead loops infinitely:

 void *
 kmem_alloc(size_t size, unsigned int __nocast flags)
 {
         int     retries = 0;
         gfp_t   lflags = kmem_flags_convert(flags);
         void    *ptr;

         do {
                 if (size < MAX_SLAB_SIZE || retries > MAX_VMALLOCS)
                         ptr = kmalloc(size, lflags);
                 else
                         ptr = __vmalloc(size, lflags, PAGE_KERNEL);
                 if (ptr || (flags & (KM_MAYFAIL|KM_NOSLEEP)))
                         return ptr;
                 if (!(++retries % 100))
                         printk(KERN_ERR "XFS: possible memory allocation "
                                         "deadlock in %s (mode:0x%x)\n",
                                         __FUNCTION__, lflags);
                 blk_congestion_wait(WRITE, HZ/50);
         } while (1);
 }

which is in essence an open-coded GFP_NOFAIL implementation. Here's what 
__GFP_NOFAIL does:

                        if (gfp_mask & __GFP_NOFAIL) {
                                blk_congestion_wait(WRITE, HZ/50);
                                goto nofail_alloc;
                        }

and since XFS makes use of KM_SLEEP in 130+ callsites, that means it is 
in essence using GFP_NOFAIL massively!

	Ingo