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SubjectRe: [PATCH 1/9] mm: Allow the [page|pfn]_mkwrite callbacks to drop the mmap_sem
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On 4/12/19 9:04 AM, Thomas Hellstrom wrote:
> Driver fault callbacks are allowed to drop the mmap_sem when expecting
> long hardware waits to avoid blocking other mm users. Allow the mkwrite
> callbacks to do the same by returning early on VM_FAULT_RETRY.
>
> In particular we want to be able to drop the mmap_sem when waiting for
> a reservation object lock on a GPU buffer object. These locks may be
> held while waiting for the GPU.
>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@surriel.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Souptick Joarder <jrdr.linux@gmail.com>
> Cc: "Jérôme Glisse" <jglisse@redhat.com>
> Cc: linux-mm@kvack.org
> Cc: linux-kernel@vger.kernel.org
>
> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>

Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>

> ---
> mm/memory.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index e11ca9dd823f..a95b4a3b1ae2 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2144,7 +2144,7 @@ static vm_fault_t do_page_mkwrite(struct vm_fault *vmf)
> ret = vmf->vma->vm_ops->page_mkwrite(vmf);
> /* Restore original flags so that caller is not surprised */
> vmf->flags = old_flags;
> - if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))
> + if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_RETRY | VM_FAULT_NOPAGE)))

A very minor nit, for consistency elsewhere in mm/memory.c,
could you make this be:
(VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)

> return ret;
> if (unlikely(!(ret & VM_FAULT_LOCKED))) {
> lock_page(page);
> @@ -2419,7 +2419,7 @@ static vm_fault_t wp_pfn_shared(struct vm_fault *vmf)
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> vmf->flags |= FAULT_FLAG_MKWRITE;
> ret = vma->vm_ops->pfn_mkwrite(vmf);
> - if (ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE))
> + if (ret & (VM_FAULT_ERROR | VM_FAULT_RETRY | VM_FAULT_NOPAGE))
> return ret;
> return finish_mkwrite_fault(vmf);
> }
> @@ -2440,7 +2440,8 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf)
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> tmp = do_page_mkwrite(vmf);
> if (unlikely(!tmp || (tmp &
> - (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
> + (VM_FAULT_ERROR | VM_FAULT_RETRY |
> + VM_FAULT_NOPAGE)))) {
> put_page(vmf->page);
> return tmp;
> }
> @@ -3494,7 +3495,8 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf)
> unlock_page(vmf->page);
> tmp = do_page_mkwrite(vmf);
> if (unlikely(!tmp ||
> - (tmp & (VM_FAULT_ERROR | VM_FAULT_NOPAGE)))) {
> + (tmp & (VM_FAULT_ERROR | VM_FAULT_RETRY |
> + VM_FAULT_NOPAGE)))) {
> put_page(vmf->page);
> return tmp;
> }
>

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