lkml.org 
[lkml]   [2019]   [Sep]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH 5/5] hugetlbfs: Limit wait time when trying to share huge PMD
    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

    \
     
     \ /
      Last update: 2019-09-12 06:40    [W:2.634 / U:0.296 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site