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SubjectRE: [PATCH 23/26] ib: Convert qib_get_user_pages() to get_user_pages_unlocked()
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> > 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()?

The old code looked like:
__qib_get_user_pages()
(broken) ulimit test
for (...)
get_user_pages()

qib_get_user_pages()
mmap_sem lock
__qib_get_user_pages()
mmap_sem() unlock

The new code is:

get_user_pages_unlocked()
mmap_sem lock
get_user_pages()
mmap_sem unlock

qib_get_user_pages()
mmap_sem lock
ulimit test and locked pages maintenance
mmap_sem unlock
for (...)
get_user_pages_unlocked()

I count an additional pair of mmap_sem transactions in the normal case.

>
> > 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...
>

This is what I had in mind:

get_user_pages_ulimit_unlocked()
mmap_sem lock
ulimit test and locked pages maintenance (from qib/ipath)
for (...)
get_user_pages_unlocked()
mmap_sem unlock

qib_get_user_pages()
get_user_pages_ulimit_unlocked()

This really pushes the code into a new wrapper common to ipath/qib and any others that might want to combine locking with ulimit enforcement.

Mike


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