On Thu, Nov 22, 2018 at 12:26:02AM -0800, Christoph Hellwig wrote: > On Wed, Nov 21, 2018 at 06:35:58PM -0800, Matthew Wilcox wrote: > > I think you should look at using the page_frag allocator here. You can > > use whatever GFP_DMA flags you like. > > So I actually tries to use page_frag to solve the XFS unaligned kmalloc > allocations problem, and I don't think it is the right hammer for this > nail (or any other nail outside of networking). > > The problem with the page_frag allocator is that it never reuses > fragments returned to the page, but only only frees the page once all > fragments are freed. This means that if you have some long(er) term > allocations you are effectively creating memory leaks. Yes, your allocations from the page_frag allocator have to have similar lifetimes. I thought that would be ideal for XFS though; as I understood the problem, these were per-IO allocations, and IOs to the same filesystem tend to take roughly the same amount of time. Sure, in an error case, some IOs will take a long time before timing out, but it should be OK to have pages unavailable during that time in these rare situations. What am I missing?
