On Tue, 6 Sep 2011, Christoph Hellwig wrote: > On Tue, Sep 06, 2011 at 11:57:00AM -0400, Mikulas Patocka wrote: > > > IS there any reason you'll need a fixed size? This is fairly similar in > > > concept to the XFS buffercache, which does perfectly well by allocation > > > memory as needed, and letting the shrinker reclaim buffers when under > > > memory pressure. > > > > It is possible to make unlimited size. --- the question: is the shrinker > > run when we exhaust vmalloc arena? > > > > dm-bufio cache uses vmalloc arena under some circumstances. On some > > architectures (for example i386), vmalloc arena is smaller than main > > memory, therefore it may overflow before main memory does. > > > > What does XFS do when vmalloc arena is exhausted? > > At this point shrinkers do not handle vmalloc space, although we could > add them. In the default configuration XFS uses very little vmalloc > space in the buffer cache - only the 8 log buffers are vmapped, and > those can't be reclaimed anyway. During log recovery or if using the > non-standard larger directory block mkfs option it can consume a larger > amount of vmalloc space, and we have run into problems because of that, > e.g. take a look at the loop around vm_map_ram() in _xfs_buf_map_pages() > that we had to add as a workaround, and the commit introducing it for > more details (a19fb380). I see --- but: shouldn't vm_map_ram() do its own cleanup and call vm_unmap_aliases() accordingly? Do you mean that any function that allocates something from vmalloc area needs to call vm_unmap_aliases() and retry in case of failure? > Just curious, why do you need the buffers to be vmapped? If we'd design > the dir2 format these days we'd make sure it is aligned in a way that > we could deal with individually mapped pages. I need it if we use larger buffers than pages. They are allocated with get_free_pages, but it is unreliable and has its own limit, so I use vmalloc as a reliable backup. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
