Re: page fault fastpath patch v2: fix race conditions, stats for 8,32 and 512 cpu SMP

[William Lee Irwin III <wli@holomorphy.com>]

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.


-- wli