On Tue, 27 Sep 2016 11:37:24 +0000 David Laight <david.lai...@aculab.com> wrote:
> From: Nicholas Piggin > > Sent: 27 September 2016 12:25 > > On Tue, 27 Sep 2016 10:44:04 +0200 > > Vlastimil Babka <vba...@suse.cz> wrote: > > > > > On 09/23/2016 06:47 PM, Jason Baron wrote: > > > > Hi, > > > > > > > > On 09/23/2016 03:24 AM, Nicholas Piggin wrote: > > > >> On Fri, 23 Sep 2016 14:42:53 +0800 > > > >> "Hillf Danton" <hillf...@alibaba-inc.com> wrote: > > > >> > > > >>>> > > > >>>> The select(2) syscall performs a kmalloc(size, GFP_KERNEL) where > > > >>>> size grows > > > >>>> with the number of fds passed. We had a customer report page > > > >>>> allocation > > > >>>> failures of order-4 for this allocation. This is a costly order, so > > > >>>> it might > > > >>>> easily fail, as the VM expects such allocation to have a lower-order > > > >>>> fallback. > > > >>>> > > > >>>> Such trivial fallback is vmalloc(), as the memory doesn't have to be > > > >>>> physically contiguous. Also the allocation is temporary for the > > > >>>> duration of the > > > >>>> syscall, so it's unlikely to stress vmalloc too much. > > > >>>> > > > >>>> Note that the poll(2) syscall seems to use a linked list of order-0 > > > >>>> pages, so > > > >>>> it doesn't need this kind of fallback. > > > >> > > > >> How about something like this? (untested) > > > > > > This pushes the limit further, but might just delay the problem. Could be > > > an > > > optimization on top if there's enough interest, though. > > > > What's your customer doing with those selects? If they care at all about > > performance, I doubt they want select to attempt order-4 allocations, fail, > > then use vmalloc :) > > If they care about performance they shouldn't be passing select() lists that > are anywhere near that large. > If the number of actual fd is small - use poll(). Right. Presumably it's some old app they're still using, no?
