Re: [PATCH 1/2] mm: Introduce a general RCU get_user_pages_fast.

[Aneesh Kumar K.V]

Steve Capper <steve.capper@xxxxxxxxxx> writes:

> On Tue, Oct 14, 2014 at 05:38:43PM +0530, Aneesh Kumar K.V wrote:
>> Steve Capper <steve.capper@xxxxxxxxxx> writes:
>> 
>> > On Tue, Oct 14, 2014 at 04:27:53PM +0530, Aneesh Kumar K.V wrote:
>> >> get_user_pages_fast attempts to pin user pages by walking the page
>> >> tables directly and avoids taking locks. Thus the walker needs to be
>> >> protected from page table pages being freed from under it, and needs
>> >> to block any THP splits.
>> >> 
>> >> One way to achieve this is to have the walker disable interrupts, and
>> >> rely on IPIs from the TLB flushing code blocking before the page table
>> >> pages are freed.
>> >> 
>> >> On some platforms we have hardware broadcast of TLB invalidations, thus
>> >> the TLB flushing code doesn't necessarily need to broadcast IPIs; and
>> >> spuriously broadcasting IPIs can hurt system performance if done too
>> >> often.
>> >> 
>> >> This problem has been solved on PowerPC and Sparc by batching up page
>> >> table pages belonging to more than one mm_user, then scheduling an
>> >> rcu_sched callback to free the pages. This RCU page table free logic
>> >> has been promoted to core code and is activated when one enables
>> >> HAVE_RCU_TABLE_FREE. Unfortunately, these architectures implement
>> >> their own get_user_pages_fast routines.
>> >> 
>> >> The RCU page table free logic coupled with a an IPI broadcast on THP
>> >> split (which is a rare event), allows one to protect a page table
>> >> walker by merely disabling the interrupts during the walk.
>> >> 
>> >> This patch provides a general RCU implementation of get_user_pages_fast
>> >> that can be used by architectures that perform hardware broadcast of
>> >> TLB invalidations.
>> >> 
>> >> It is based heavily on the PowerPC implementation.
>> >> 
>> >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
>> >> ---
>> >> 
>> >> NOTE: I kept the description of patch as it is and also retained the documentation.
>> >> I also dropped the tested-by and other SOB, because i was not sure whether they want
>> >> to be blamed for the bugs here. Please feel free to update.
>> >
>> > Hi Aneesh,
>> > Thank you for coding this up.
>> >
>> > I've compiled and briefly tested this on arm (with and without LPAE),
>> > and arm64. I ran a custom futex on THP tail test, and this passed.
>> > I'll test this a little more aggressively with ltp.
>> >
>> > I think Linus has already pulled in the RCU gup I posted, could you
>> > please instead write a patch against?
>> 
>> Will do that. 
>> 
>> > 2667f50 mm: introduce a general RCU get_user_pages_fast()
>> 
>> In that patch don't you need to check the _PAGE_PRESENT details in case
>> of gup_huge_pmd ?
>> 
>
> Before calling gup_huge_pmd we check for pmd_none. On ARM we have:
> #define pmd_none(pmd)		(!pmd_val(pmd))
> #define pmd_present(pmd)	(pmd_val(pmd))
>
> So we know when we enter gup_huge_pmd that "orig" represents a pmd that
> is present.
>
> On PowerPC I see the definitions are a little different for
> pgtable-ppc32.h:
> #define pmd_present(pmd)	(pmd_val(pmd) & _PMD_PRESENT_MASK)
>
> A pmd_present() could be added to the beginning of gup_huge_pmd safely
> if this is needed for PowerPC.
>

for the huge pmd case, that is pte in pmd we may really want to check
_PAGE_PRESENT bit right ?

-aneesh

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