>>>>> "Mike" == Mike Snitzer <snitzer@xxxxxxxxxx> writes: Mike> On Tue, May 01 2018 at 8:36pm -0400, Mike> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote: >> On Tue, 24 Apr 2018 12:33:01 -0400 (EDT) Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: >> >> > >> > >> > On Tue, 24 Apr 2018, Michal Hocko wrote: >> > >> > > On Tue 24-04-18 11:30:40, Mikulas Patocka wrote: >> > > > >> > > > >> > > > On Tue, 24 Apr 2018, Michal Hocko wrote: >> > > > >> > > > > On Mon 23-04-18 20:25:15, Mikulas Patocka wrote: >> > > > > >> > > > > > Fixing __vmalloc code >> > > > > > is easy and it doesn't require cooperation with maintainers. >> > > > > >> > > > > But it is a hack against the intention of the scope api. >> > > > >> > > > It is not! >> > > >> > > This discussion simply doesn't make much sense it seems. The scope API >> > > is to document the scope of the reclaim recursion critical section. That >> > > certainly is not a utility function like vmalloc. >> > >> > That 15-line __vmalloc bugfix doesn't prevent you (or any other kernel >> > developer) from converting the code to the scope API. You make nonsensical >> > excuses. >> > >> >> Fun thread! >> >> Winding back to the original problem, I'd state it as >> >> - Caller uses kvmalloc() but passes the address into vmalloc-naive >> DMA API and >> >> - Caller uses kvmalloc() but passes the address into kfree() >> >> Yes? Mike> I think so. >> If so, then... >> >> Is there a way in which, in the kvmalloc-called-kmalloc path, we can >> tag the slab-allocated memory with a "this memory was allocated with >> kvmalloc()" flag? I *think* there's extra per-object storage available >> with suitable slab/slub debugging options? Perhaps we could steal one >> bit from the redzone, dunno. >> >> If so then we can >> >> a) set that flag in kvmalloc() if the kmalloc() call succeeded >> >> b) check for that flag in the DMA code, WARN if it is set. >> >> c) in kvfree(), clear that flag before calling kfree() >> >> d) in kfree(), check for that flag and go WARN() if set. >> >> So both potential bugs are detected all the time, dependent upon >> CONFIG_SLUB_DEBUG (and perhaps other slub config options). Mike> Thanks Andrew, definitely the most sane proposal I've seen to resolve Mike> this. Cuts to the heart of the issue I think, and seems pretty sane. Should the WARN be rate limited as well? John
