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    SubjectRe: [PATCH] mm/hugetlb: prevent hugetlb VMA to be misaligned
    On Tue 20-03-18 14:35:28, Mike Kravetz wrote:
    > On 03/20/2018 02:26 PM, Mike Kravetz wrote:
    > > Thanks Laurent!
    > >
    > > This bug was introduced by 31383c6865a5. Dan's changes for 31383c6865a5
    > > seem pretty straight forward. It simply replaces an explicit check when
    > > splitting a vma to a new vm_ops split callout. Unfortunately, mappings
    > > created via shmget/shmat have their vm_ops replaced. Therefore, this
    > > split callout is never made.
    > >
    > > The shm vm_ops do indirectly call the original vm_ops routines as needed.
    > > Therefore, I would suggest a patch something like the following instead.
    > > If we move forward with the patch, we should include Laurent's BUG output
    > > and perhaps test program in the commit message.
    >
    > Sorry, patch in previous mail was a mess
    >
    > >From 7a19414319c7937fd2757c27f936258f16c1f61d Mon Sep 17 00:00:00 2001
    > From: Mike Kravetz <mike.kravetz@oracle.com>
    > Date: Tue, 20 Mar 2018 13:56:57 -0700
    > Subject: [PATCH] shm: add split function to shm_vm_ops
    >
    > The split function was added to vm_operations_struct to determine
    > if a mapping can be split. This was mostly for device-dax and
    > hugetlbfs mappings which have specific alignment constraints.
    >
    > mappings initiated via shmget/shmat have their original vm_ops
    > overwritten with shm_vm_ops. shm_vm_ops functions will call back
    > to the original vm_ops if needed. Add such a split function.
    >
    > Fixes: 31383c6865a5 ("mm, hugetlbfs: introduce ->split() to vm_operations_struct)
    > Reported by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
    > Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>

    Yes this looks much better than the original hugetlb specific code in
    the generic vma code.

    Please add the original VM_BUG_ON report to the changelog

    Cc: stable
    Acked-by: Michal Hocko <mhocko@suse.com>

    > ---
    > ipc/shm.c | 12 ++++++++++++
    > 1 file changed, 12 insertions(+)
    >
    > diff --git a/ipc/shm.c b/ipc/shm.c
    > index 7acda23430aa..50e88fc060b1 100644
    > --- a/ipc/shm.c
    > +++ b/ipc/shm.c
    > @@ -386,6 +386,17 @@ static int shm_fault(struct vm_fault *vmf)
    > return sfd->vm_ops->fault(vmf);
    > }
    >
    > +static int shm_split(struct vm_area_struct *vma, unsigned long addr)
    > +{
    > + struct file *file = vma->vm_file;
    > + struct shm_file_data *sfd = shm_file_data(file);
    > +
    > + if (sfd->vm_ops && sfd->vm_ops->split)
    > + return sfd->vm_ops->split(vma, addr);
    > +
    > + return 0;
    > +}
    > +
    > #ifdef CONFIG_NUMA
    > static int shm_set_policy(struct vm_area_struct *vma, struct mempolicy *new)
    > {
    > @@ -510,6 +521,7 @@ static const struct vm_operations_struct shm_vm_ops = {
    > .open = shm_open, /* callback for a new vm-area open */
    > .close = shm_close, /* callback for when the vm-area is released */
    > .fault = shm_fault,
    > + .split = shm_split,
    > #if defined(CONFIG_NUMA)
    > .set_policy = shm_set_policy,
    > .get_policy = shm_get_policy,
    > --
    > 2.13.6

    --
    Michal Hocko
    SUSE Labs

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