From: Linus Torvalds <torvalds@g5.osdl.org>
Date: Mon, 1 Aug 2005 18:14:49 +0000 (-0700)
Subject: Fix get_user_pages() race for write access
X-Git-Tag: v2.6.13-rc5~32
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4ceb5db9757a;p=pandora-kernel.git

Fix get_user_pages() race for write access

There's no real guarantee that handle_mm_fault() will always be able to
break a COW situation - if an update from another thread ends up
modifying the page table some way, handle_mm_fault() may end up
requiring us to re-try the operation.

That's normally fine, but get_user_pages() ended up re-trying it as a
read, and thus a write access could in theory end up losing the dirty
bit or be done on a page that had not been properly COW'ed.

This makes get_user_pages() always retry write accesses as write
accesses by making "follow_page()" require that a writable follow has
the dirty bit set.  That simplifies the code and solves the race: if the
COW break fails for some reason, we'll just loop around and try again.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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