On Mon, Feb 25, 2019 at 04:46:25PM +0800, Ming Lei wrote: > On Mon, Feb 25, 2019 at 03:36:48PM +1100, Dave Chinner wrote: > > On Mon, Feb 25, 2019 at 12:09:04PM +0800, Ming Lei wrote: > > > XFS uses kmalloc() to allocate sector sized IO buffer. > > .... > > > Use page_frag_alloc() to allocate the sector sized buffer, then the > > > above issue can be fixed because offset_in_page of allocated buffer > > > is always sector aligned. > > > > Didn't we already reject this approach because page frags cannot be > > I remembered there is this kind of issue mentioned, but just not found > the details, so post out the patch for restarting the discussion. > > > reused and that pages allocated to the frag pool are pinned in > > memory until all fragments allocated on the page have been freed? > > Yes, that is one problem. But if one page is consumed, sooner or later, > all fragments will be freed, then the page becomes available again. > > > > > i.e. when we consider 64k page machines and 4k block sizes (i.e. > > default config), every single metadata allocation is a sub-page > > allocation and so will use this new page frag mechanism. IOWs, it > > will result in fragmenting memory severely and typical memory > > reclaim not being able to fix it because the metadata that pins each > > page is largely unreclaimable... > > It can be an issue in case of IO timeout & retry. The worst case is still not worse than allocating single page for sub-page IO, which should be used on other file systems under the same situation, I guess. thanks, Ming
