On Tue 03-04-18 08:23:48, Steven Rostedt wrote: > On Tue, 3 Apr 2018 14:16:14 +0200 > Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > This came up because there's scripts or programs that set the size of > > > the ring buffer. The complaint was that the application would just set > > > the size to something bigger than what was available and cause an OOM > > > killing other applications. The final solution is to simply check the > > > available memory before allocating the ring buffer: > > > > > > /* Check if the available memory is there first */ > > > i = si_mem_available(); > > > if (i < nr_pages) > > > return -ENOMEM; > > > > > > And it works well. > > > > Except that it doesn't work. si_mem_available is not really suitable for > > any allocation estimations. Its only purpose is to provide a very rough > > estimation for userspace. Any other use is basically abuse. The > > situation can change really quickly. Really it is really hard to be > > clever here with the volatility the memory allocations can cause. > > OK, then what do you suggest? Because currently, it appears to work. A > rough estimate may be good enough. > > If we use NORETRY, then we have those that complain that we do not try > hard enough to reclaim memory. If we use RETRY_MAYFAIL we have this > issue of taking up all memory before we get what we want. Just try to do what admin asks for and trust it will not try to shoot his foot? I mean there are other ways admin can shoot the machine down. Being clever is OK if it doesn't add a tricky code. And relying on si_mem_available is definitely tricky and obscure. > Perhaps I should try to allocate a large group of pages with > RETRY_MAYFAIL, and if that fails go back to NORETRY, with the thinking > that the large allocation may reclaim some memory that would allow the > NORETRY to succeed with smaller allocations (one page at a time)? That again relies on a subtle dependencies of the current implementation. So I would rather ask whether this is something that really deserves special treatment. If admin asks for a buffer of a certain size then try to do so. If we get OOM then bad luck you cannot get large memory buffers for free... -- Michal Hocko SUSE Labs
