On Thu 05-04-18 09:15:01, Matthew Wilcox wrote: > On Thu, Apr 05, 2018 at 05:32:40PM +0200, Michal Hocko wrote: > > On Thu 05-04-18 08:13:59, Matthew Wilcox wrote: > > > Argh. The comment confused me. OK, now I've read the source and > > > understand that GFP_KERNEL | __GFP_RETRY_MAYFAIL tries exactly as hard > > > as GFP_KERNEL *except* that it won't cause OOM itself. But any other > > > simultaneous GFP_KERNEL allocation without __GFP_RETRY_MAYFAIL will > > > cause an OOM. (And that's why we're having a conversation) > > > > Well, I can udnerstand how this can be confusing. The all confusion > > boils down to the small-never-fails semantic we have. So all reclaim > > modificators (__GFP_NOFAIL, __GFP_RETRY_MAYFAIL, __GFP_NORETRY) only > > modify the _default_ behavior. > > Now that I understand the flag, I'll try to write a more clear > explanation.
Good luck with that. It took me several iterations to land with the current state. It is quite hard to be not misleading yet understandable. > > > That's a problem because we have places in the kernel that call > > > kv[zm]alloc(very_large_size, GFP_KERNEL), and that will turn into vmalloc, > > > which will do the exact same thing, only it will trigger OOM all by itself > > > (assuming the largest free chunk of address space in the vmalloc area > > > is larger than the amount of free memory). > > > > well, hardcoded GFP_KERNEL from vmalloc guts is yet another, ehm, > > herritage that you are not so proud of. > > Certainly not, but that's not what I'm concerned about; I'm concerned > about the allocation of the pages, not the allocation of the array > containing the page pointers. Those pages will use the gfp flag you give to vmalloc IIRC. It is page tables that are GFP_KERNEL unconditionally. > > > We could also have a GFP flag that says to only succeed if we're further > > > above the existing watermark than normal. __GFP_LOW (==ALLOC_LOW), > > > if you like. That would give us the desired behaviour of trying all of > > > the reclaim methods that GFP_KERNEL would, but not being able to exhaust > > > all the memory that GFP_KERNEL allocations would take. > > > > Well, I would be really careful with yet another gfp mask. They are so > > incredibly hard to define properly and then people kinda tend to screw > > your best intentions with their usecases ;) > > Failing on low wmark is very close to __GFP_NORETRY or even > > __GFP_NOWAIT, btw. So let's try to not overthink this... > > Oh, indeed. We must be able to clearly communicate to users when they > should use this flag. I have in mind something like this: > > * __GFP_HIGH indicates that the caller is high-priority and that granting > * the request is necessary before the system can make forward progress. > * For example, creating an IO context to clean pages. > * > * __GFP_LOW indicates that the caller is low-priority and that it should > * not be allocated pages that would cause the system to get into an > * out-of-memory situation. For example, allocating multiple individual > * pages in order to satisfy a larger request. So how exactly the low fits into GFP_NOWAIT, GFP_NORETRY and GFP_RETRY_MAFAIL? We _do_have several levels of how hard to try and we have users relying on that. And do not forget about costly vs. non-costly sizes. That being said, we should not hijack this thread more and further enhancements should be discussed separatelly. I am all for making the whole allocation api less obscure but keep in mind that we have really hard historical restrictions. -- Michal Hocko SUSE Labs
