On Fri, Oct 02, 2020 at 09:06:24AM +0100, Mel Gorman wrote: > On Thu, Oct 01, 2020 at 09:26:26PM +0200, Uladzislau Rezki wrote: > > > > > > No, I meant going back to idea of new gfp flag, but adjust the implementation in > > > the allocator (different from what you posted in previous version) so that it > > > only looks at the flag after it tries to allocate from pcplist and finds out > > > it's empty. So, no inventing of new page allocator entry points or checks such > > > as the one you wrote above, but adding the new gfp flag in a way that it doesn't > > > affect existing fast paths. > > > > > OK. Now i see. Please have a look below at the patch, so we fully understand > > each other. If that is something that is close to your view or not: > > > > <snip> > > t a/include/linux/gfp.h b/include/linux/gfp.h > > index c603237e006c..7e613560a502 100644 > > --- a/include/linux/gfp.h > > +++ b/include/linux/gfp.h > > @@ -39,8 +39,9 @@ struct vm_area_struct; > > #define ___GFP_HARDWALL 0x100000u > > #define ___GFP_THISNODE 0x200000u > > #define ___GFP_ACCOUNT 0x400000u > > +#define ___GFP_NO_LOCKS 0x800000u > > #ifdef CONFIG_LOCKDEP > > -#define ___GFP_NOLOCKDEP 0x800000u > > +#define ___GFP_NOLOCKDEP 0x1000000u > > #else > > #define ___GFP_NOLOCKDEP 0 > > #endif > > @@ -215,16 +216,22 @@ struct vm_area_struct; > > * %__GFP_COMP address compound page metadata. > > * > > * %__GFP_ZERO returns a zeroed page on success. > > + * > > + * %__GFP_NO_LOCKS order-0 allocation without sleepable-locks. > > + * It obtains a page from the per-cpu-list and considered as > > + * lock-less. No other actions are performed, thus it returns > > + * NULL if per-cpu-list is empty. > > */ > > #define __GFP_NOWARN ((__force gfp_t)___GFP_NOWARN) > > #define __GFP_COMP ((__force gfp_t)___GFP_COMP) > > #define __GFP_ZERO ((__force gfp_t)___GFP_ZERO) > > +#define __GFP_NO_LOCKS ((__force gfp_t)___GFP_NO_LOCKS) > > > > I'm not a fan of the GFP flag approach simply because we've had cases > before where GFP flags were used in inappropriate contexts like > __GFP_MEMALLOC which led to a surprising amount of bugs, particularly > from out-of-tree drivers but also in-tree drivers. Of course, there > are limited GFP flags available too but at least the comment should > be as robust as possible. Maybe something like > > * %__GFP_NO_LOCKS attempts order-0 allocation without sleepable-locks. It > * attempts to obtain a page without acquiring any spinlocks. This > * should only be used in a context where the holder holds a > * raw_spin_lock that cannot be released for the allocation request. > * This may be necessary in PREEMPT_RT kernels where a > * raw_spin_lock is held which does not sleep tries to acquire a > * spin_lock that can sleep with PREEMPT_RT. This should not be > * confused with GFP_ATOMIC contexts. Like atomic allocation > * requests, there is no guarantee a page will be returned and > * the caller must be able to deal with allocation failures. > * The risk of allocation failure is higher than using GFP_ATOMIC. > > It's verbose but it would be hard to misinterpret. I think we're > going to go through a period of time before people get familiar > with PREEMPT_RT-related hazards as various comments that were > true are going to be misleading for a while. > Yep, it should be properly documented for sure. Including new GFP_NOWAIT limitations, same as GFP_ATOMIC once you mentioned. Thanks! -- Vlad Rezki
