Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD

From: Davidlohr Bueso
Date: Thu Sep 12 2019 - 00:40:23 EST

On Wed, 11 Sep 2019, Matthew Wilcox wrote:

On Wed, Sep 11, 2019 at 08:26:52PM -0700, Mike Kravetz wrote:
All this got me wondering if we really need to take i_mmap_rwsem in write
mode here. We are not changing the tree, only traversing it looking for
a suitable vma.

Unless I am missing something, the hugetlb code only ever takes the semaphore
in write mode; never read. Could this have been the result of changing the
tree semaphore to read/write? Instead of analyzing all the code, the easiest
and safest thing would have been to take all accesses in write mode.

I was wondering the same thing. It was changed here:

commit 83cde9e8ba95d180eaefefe834958fbf7008cf39
Author: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Date: Fri Dec 12 16:54:21 2014 -0800

mm: use new helper functions around the i_mmap_mutex

Convert all open coded mutex_lock/unlock calls to the
i_mmap_[lock/unlock]_write() helpers.

and a subsequent patch said:

This conversion is straightforward. For now, all users take the write

There were subsequent patches which changed a few places

but I don't know why this one wasn't changed.

I cannot recall why huge_pmd_share() was not changed along with the other
callers that don't modify the interval tree. By looking at the function,
I agree that this could be shared, in fact this lock is much less involved
than it's anon_vma counterpart, last I checked (perhaps with the exception
of take_rmap_locks().

(I was also wondering about caching a potentially sharable page table
in the address_space to avoid having to walk the VMA tree at all if that
one happened to be sharable).

I also think that the right solution is within the mm instead of adding
a new api to rwsem and the extra complexity/overhead to osq _just_ for this
case. We've managed to not need timeout extensions in our locking primitives
thus far, which is a good thing imo.