On Thu 26-11-15 11:16:24, Vladimir Davydov wrote: > On Wed, Nov 25, 2015 at 07:27:57PM +0300, Vladimir Davydov wrote: > > On Wed, Nov 25, 2015 at 04:45:13PM +0100, Vlastimil Babka wrote: > > > On 11/25/2015 04:36 PM, Vladimir Davydov wrote: > > > > Block device drivers often hand off io request processing to kernel > > > > threads (example: device mapper). If such a thread calls kmalloc, it can > > > > dive into direct reclaim path and end up waiting for too_many_isolated > > > > to return false, blocking writeback. This can lead to a dead lock if the > > > > > > Shouldn't such allocation lack __GFP_IO to prevent this and other kinds of > > > deadlocks? And/or have mempools? > > > > Not necessarily. loopback is an example: it can call > > grab_cache_write_begin -> add_to_page_cache_lru with GFP_KERNEL. AFAIR loop driver reduces the gfp_maks via inode mapping. > Anyway, kthreads that use GFP_NOIO and/or mempool aren't safe either, > because it isn't an allocation context problem: the reclaimer locks up > not because it tries to take an fs/io lock the caller holds, but because > it waits for isolated pages to be put back, which will never happen, > since processes that isolated them depend on the kthread making > progress. This is purely a reclaimer heuristic, which kmalloc users are > not aware of. > > My point is that, in contrast to userspace processes, it is dangerous to > throttle kthreads in the reclaimer, because they might be responsible > for reclaimer progress (e.g. performing writeback). Wouldn't it be better if your writeback kthread did PF_MEMALLOC/__GFP_MEMALLOC instead because it is in fact a reclaimer so it even get to the reclaim. There way too many allocations done from the kernel thread context to be not throttled (just look at worker threads). -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>