On Wed, 24 Feb 2016 18:59:21 +0300 "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > Previously, __split_huge_page_splitting() required serialization against > gup_fast to make sure nobody can obtain new reference to the page after > __split_huge_page_splitting() returns. This was a way to stabilize page > references before starting to distribute them from head page to tail > pages. > > With new refcounting, we don't care about this. Splitting PMD is now > decoupled from splitting underlying compound page. It's okay to get new > pins after split_huge_pmd(). To stabilize page references during > split_huge_page() we rely on setting up migration entries once all > pmds are split into page tables. > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > --- > mm/gup.c | 11 +++-------- > mm/huge_memory.c | 7 +++---- > 2 files changed, 6 insertions(+), 12 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 7bf19ffa2199..2f528fce3a62 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -1087,8 +1087,7 @@ struct page *get_dump_page(unsigned long addr) > * > * 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 should > - * block any THP splits. > + * protected from page table pages being freed from under it. > * > * 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 > @@ -1097,9 +1096,8 @@ struct page *get_dump_page(unsigned long addr) > * > * Another way to achieve this is to batch up page table containing pages > * belonging to more than one mm_user, then rcu_sched a callback to free those > - * pages. Disabling interrupts will allow the fast_gup walker to both block > - * the rcu_sched callback, and an IPI that we broadcast for splitting THPs > - * (which is a relatively rare event). The code below adopts this strategy. > + * pages. Disabling interrupts will allow the fast_gup walker to block > + * the rcu_sched callback. The code below adopts this strategy. > * > * Before activating this code, please be aware that the following assumptions > * are currently made: > @@ -1391,9 +1389,6 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write, > * With interrupts disabled, we block page table pages from being > * freed from under us. See mmu_gather_tlb in asm-generic/tlb.h > * for more details. > - * > - * We do not adopt an rcu_read_lock(.) here as we also want to > - * block IPIs that come from THPs splitting. > */ Hmm, now that the IPI from THP splitting is not needed anymore, this comment would suggest that we could use rcu_read_lock(_sched) for fast_gup, instead of keeping the (probably more expensive) IRQ enable/ disable. That should be enough to synchronize against the call_rcu_sched() from the batched tlb_table_flush, right? So, if the comment is correct, removing it would also remove the hint that we could use RCU instead if IRQ enable/disable. > > local_irq_save(flags); > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index e10a4fee88d2..846fe173e04b 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -2930,10 +2930,9 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, > * for the same virtual address to be loaded simultaneously. So instead > * of doing "pmd_populate(); flush_pmd_tlb_range();" we first mark the > * current pmd notpresent (atomically because here the pmd_trans_huge > - * and pmd_trans_splitting must remain set at all times on the pmd > - * until the split is complete for this pmd), then we flush the SMP TLB > - * and finally we write the non-huge version of the pmd entry with > - * pmd_populate. > + * must remain set at all times on the pmd until the split_huge_pmd() > + * is complete, then we flush the SMP TLB and finally we write the > + * non-huge version of the pmd entry with pmd_populate. > */ > pmdp_invalidate(vma, haddr, pmd); > pmd_populate(mm, pmd, pgtable); -- 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>