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SubjectRe: [PATCH 23/26] ib: Convert qib_get_user_pages() to get_user_pages_unlocked()
On Fri 04-10-13 13:52:49, Marciniszyn, Mike wrote:
> > Convert qib_get_user_pages() to use get_user_pages_unlocked(). This
> > shortens the section where we hold mmap_sem for writing and also removes
> > the knowledge about get_user_pages() locking from ipath driver. We also fix
> > a bug in testing pinned number of pages when changing the code.
> >
>
> This patch and the sibling ipath patch will nominally take the mmap_sem
> twice where the old routine only took it once. This is a performance
> issue.
It will take mmap_sem only once during normal operation. Only if
get_user_pages_unlocked() fail, we have to take mmap_sem again to undo
the change of mm->pinned_vm.

> Is the intent here to deprecate get_user_pages()?
Well, as much as I'd like to, there are really places in mm code which
need to call get_user_pages() while holding mmap_sem to be able to inspect
corresponding vmas etc. So I want to reduce get_user_pages() use as much as
possible but I'm not really hoping in completely removing it.

> I agree, the old code's lock limit test is broke and needs to be fixed.
> I like the elimination of the silly wrapper routine!
>
> Could the lock limit test be pushed into another version of the wrapper
> so that there is only one set of mmap_sem transactions?
I'm sorry, I don't understand what you mean here...

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR


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