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    SubjectRe: [RFC-PATCH 2/4] mm: Add __rcu_alloc_page_lockless() func.
    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

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