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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>