On Tue 06-06-17 11:04:01, Wei Yang wrote: > On Mon, Jun 05, 2017 at 08:43:43AM +0200, Michal Hocko wrote: > >On Sat 03-06-17 10:24:40, Wei Yang wrote: > >> Hi, Michal > >> > >> Just go through your patch. > >> > >> I have one question and one suggestion as below. > >> > >> One suggestion: > >> > >> This patch does two things to me: > >> 1. Replace __GFP_REPEAT with __GFP_RETRY_MAYFAIL > >> 2. Adjust the logic in page_alloc to provide the middle semantic > >> > >> My suggestion is to split these two task into two patches, so that readers > >> could catch your fundamental logic change easily. > > > >Well, the rename and the change is intentionally tight together. My > >previous patches have removed all __GFP_REPEAT users for low order > >requests which didn't have any implemented semantic. So as of now we > >should only have those users which semantic will not change. I do not > >add any new low order user in this patch so it in fact doesn't change > >any existing semnatic. > > > >> > >> On Tue, Mar 07, 2017 at 04:48:41PM +0100, Michal Hocko wrote: > >> >From: Michal Hocko <[email protected]> > >[...] > >> >@@ -3776,9 +3784,9 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int > >> >order, > >> > > >> > /* > >> > * Do not retry costly high order allocations unless they are > >> >- * __GFP_REPEAT > >> >+ * __GFP_RETRY_MAYFAIL > >> > */ > >> >- if (order > PAGE_ALLOC_COSTLY_ORDER && !(gfp_mask & __GFP_REPEAT)) > >> >+ if (order > PAGE_ALLOC_COSTLY_ORDER && !(gfp_mask & > >> >__GFP_RETRY_MAYFAIL)) > >> > goto nopage; > >> > >> One question: > >> > >> From your change log, it mentions will provide the same semantic for > >> !costly > >> allocations. While the logic here is the same as before. > >> > >> For a !costly allocation with __GFP_REPEAT flag, the difference after this > >> patch is no OOM will be invoked, while it will still continue in the loop. > > > >Not really. There are two things. The above will shortcut retrying if > >there is _no_ __GFP_RETRY_MAYFAIL. If the flags _is_ specified we will > >back of in __alloc_pages_may_oom. > > > >> Maybe I don't catch your point in this message: > >> > >> __GFP_REPEAT was designed to allow retry-but-eventually-fail semantic to > >> the page allocator. This has been true but only for allocations requests > >> larger than PAGE_ALLOC_COSTLY_ORDER. It has been always ignored for > >> smaller sizes. This is a bit unfortunate because there is no way to > >> express the same semantic for those requests and they are considered too > >> important to fail so they might end up looping in the page allocator for > >> ever, similarly to GFP_NOFAIL requests. > >> > >> I thought you will provide the same semantic to !costly allocation, or I > >> misunderstand? > > > >yes and that is the case. __alloc_pages_may_oom will back off before OOM > >killer is invoked and the allocator slow path will fail because > >did_some_progress == 0; > > Thanks for your explanation. > > So same "semantic" doesn't mean same "behavior". > 1. costly allocations will pick up the shut cut
yes and there are no such allocations yet (based on my previous cleanups) > 2. !costly allocations will try something more but finally fail without > invoking OOM. no, the behavior will not change for those. > Hope this time I catch your point. > > BTW, did_some_progress mostly means the OOM works to me. Are there some other > important situations when did_some_progress is set to 1? Yes e.g. for GFP_NOFS when we cannot really invoke the OOM killer yet we cannot fail the allocation. -- Michal Hocko SUSE Labs
