On Mo, 2019-05-20 at 07:23 -0700, Christoph Hellwig wrote: > On Mon, May 20, 2019 at 10:16:57AM -0400, Alan Stern wrote: > > What if the allocation requires the kernel to swap some old pages out > > to the backing store, but the backing store is on the device that the > > driver is managing? The swap can't take place until the current I/O > > operation is complete (assuming the driver can handle only one I/O > > operation at a time), and the current operation can't complete until > > the old pages are swapped out. Result: deadlock. > > > > Isn't that the whole reason for using GFP_NOIO in the first place? > > It is, or rather was. As it has been incredibly painful to wire > up the gfp_t argument through some callstacks, most notably the > vmalloc allocator which is used by a lot of the DMA allocators on > non-coherent platforms, we now have the memalloc_noio_save and > memalloc_nofs_save functions that mark a thread as not beeing to > go into I/O / FS reclaim. So even if you use GFP_KERNEL you will > not dip into reclaim with those flags set on the thread. OK, but this leaves a question open. Will the GFP_NOIO actually hurt, if it is used after memalloc_noio_save()? Regards Oliver
