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. Cheers, -- Steve -- 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>