Re: [RFT] [PATCH] kanoj-mm1-2.2.5 ia32 big memory patch

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Mon, 10 May 1999 10:33:59 -0700 (PDT), kanoj@google.engr.sgi.com
(Kanoj Sarcar) said:

> There are probably a lot of problems with the code as it stands
> today. Reviewers, please let me know of any possible improvements.
> Any ideas on how to improve the uaccess performance will also be
> greatly appreciated. Testers, your input will be most valuable.

On a first scan one thing in particular jumped out:

+/*
+ * validate in a user page, so that the kernel can use the kernel direct
+ * mapped vaddr for the physical page to access user data. This locking
+ * relies on the fact that the caller has kernel_lock held, which restricts
+ * kswapd (or anyone else looking for a free page) from running and stealing 
+ * pages. By the same token, grabbing mmap_sem is not needed. 
+ */

Unfortunately, mmap_sem _is_ needed here.  Both find_extend_vma and
handle_mm_fault need it.  You can't modify or block while scanning the
vma list without it, or you risk breaking things in threaded
applications (for example, taking a page fault in handle_mm_fault
without it can be nasty if you are in the middle of a munmap at the
time).

--Stephen
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