Messages in this thread | | | Date | Wed, 18 Aug 2004 17:01:51 -0700 | From | William Lee Irwin III <> | Subject | Re: page fault fastpath patch v2: fix race conditions, stats for 8,32 and 512 cpu SMP |
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William Lee Irwin III wrote: >> It also protects against vma tree modifications in mainline, but rmap.c >> shouldn't need it for vmas anymore, as the vma is rooted to the spot by >> mapping->i_shared_lock for file pages and anon_vma->lock for anonymous.
On Wed, Aug 18, 2004 at 07:50:21PM -0400, Rajesh Venkatasubramanian wrote: > If I am reading the code correctly, then without page_table_lock > in page_referenced_one(), we can race with exit_mmap() and page > table pages can be freed under us.
exit_mmap() has removed the vma from ->i_mmap and ->mmap prior to unmapping the pages, so this should be safe unless that operation can be caught while it's in progress.
William Lee Irwin III wrote: >> Fortunately, spare bits aren't strictly necessary, and neither is >> cmpxchg. A single invalid value can serve in place of a bitflag. When >> using such an invalid value, just xchg()'ing it and looping when the >> invalid value is seen should suffice. This holds more generally for all >> radix trees, not just pagetables, and happily xchg() or emulation >> thereof is required by core code for all arches.
On Wed, Aug 18, 2004 at 07:50:21PM -0400, Rajesh Venkatasubramanian wrote: > Good point. > Another solution may be to use the unused bytes (->lru or > ->private) in page table "struct page" as bit_spin_locks. We can > use a single bit to protect a small set of ptes (8, 16, or 32).
In general the bitwise operations are more expensive than ordinary spinlocks, and a separately-allocated spinlock (not necessarily kmalloc()'d, sitting in struct page also counts, that is, separate from the pte) introduces another cacheline to be touched where with in-place locking of the pte only the pte's cacheline is needed.
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