On Fri 17-05-19 15:18:19, Alexander Potapenko wrote: > On Fri, May 17, 2019 at 2:59 PM Michal this flag Hocko > <mhocko@xxxxxxxxxx> wrote: > > > > [It would be great to keep people involved in the previous version in the > > CC list] > Yes, I've been trying to keep everyone in the loop, but your email > fell through the cracks. > Sorry about that. No problem > > On Tue 14-05-19 16:35:36, Alexander Potapenko wrote: > > > When passed to an allocator (either pagealloc or SL[AOU]B), > > > __GFP_NO_AUTOINIT tells it to not initialize the requested memory if the > > > init_on_alloc boot option is enabled. This can be useful in the cases > > > newly allocated memory is going to be initialized by the caller right > > > away. > > > > > > __GFP_NO_AUTOINIT doesn't affect init_on_free behavior, except for SLOB, > > > where init_on_free implies init_on_alloc. > > > > > > __GFP_NO_AUTOINIT basically defeats the hardening against information > > > leaks provided by init_on_alloc, so one should use it with caution. > > > > > > This patch also adds __GFP_NO_AUTOINIT to alloc_pages() calls in SL[AOU]B. > > > Doing so is safe, because the heap allocators initialize the pages they > > > receive before passing memory to the callers. > > > > I still do not like the idea of a new gfp flag as explained in the > > previous email. People will simply use it incorectly or arbitrarily. > > We have that juicy experience from the past. > > Just to preserve some context, here's the previous email: > https://patchwork.kernel.org/patch/10907595/ > (plus the patch removing GFP_TEMPORARY for the curious ones: > https://lwn.net/Articles/729145/) Not only. GFP_REPEAT being another one and probably others I cannot remember from the top of my head. > > Freeing a memory is an opt-in feature and the slab allocator can already > > tell many (with constructor or GFP_ZERO) do not need it. > Sorry, I didn't understand this piece. Could you please elaborate? The allocator can assume that caches with a constructor will initialize the object so additional zeroying is not needed. GFP_ZERO should be self explanatory. > > So can we go without this gfp thing and see whether somebody actually > > finds a performance problem with the feature enabled and think about > > what can we do about it rather than add this maint. nightmare from the > > very beginning? > > There were two reasons to introduce this flag initially. > The first was double initialization of pages allocated for SLUB. Could you elaborate please? > However the benchmark results provided in this and the previous patch > don't show any noticeable difference - most certainly because the cost > of initializing the page is amortized. > The second one was to fine-tune hackbench, for which the slowdown > drops by a factor of 2. > But optimizing a mitigation for certain benchmarks is a questionable > measure, so maybe we could really go without it. Agreed. Over optimization based on an artificial workloads tend to be dubious IMHO. -- Michal Hocko SUSE Labs
