Messages in this thread | | | Date | Wed, 11 Sep 2019 21:40:02 -0700 | From | Davidlohr Bueso <> | Subject | Re: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD |
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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@stgolabs.net> >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 > lock. > >There were subsequent patches which changed a few places >c8475d144abb1e62958cc5ec281d2a9e161c1946 >1acf2e040721564d579297646862b8ea3dd4511b >d28eb9c861f41aa2af4cfcc5eeeddff42b13d31e >874bfcaf79e39135cd31e1cfc9265cf5222d1ec3 >3dec0ba0be6a532cac949e02b853021bf6d57dad > >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.
Thanks, Davidlohr
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