Hi Punit, Thanks for reviewing this series. On 30/04/2018 20:47, Punit Agrawal wrote: > Hi Laurent, > > One nitpick below. > > On Tue, Apr 17, 2018 at 3:33 PM, Laurent Dufour > <ldufour@xxxxxxxxxxxxxxxxxx> wrote: >> This change is inspired by the Peter's proposal patch [1] which was >> protecting the VMA using SRCU. Unfortunately, SRCU is not scaling well in >> that particular case, and it is introducing major performance degradation >> due to excessive scheduling operations. >> >> To allow access to the mm_rb tree without grabbing the mmap_sem, this patch >> is protecting it access using a rwlock. As the mm_rb tree is a O(log n) >> search it is safe to protect it using such a lock. The VMA cache is not >> protected by the new rwlock and it should not be used without holding the >> mmap_sem. >> >> To allow the picked VMA structure to be used once the rwlock is released, a >> use count is added to the VMA structure. When the VMA is allocated it is >> set to 1. Each time the VMA is picked with the rwlock held its use count >> is incremented. Each time the VMA is released it is decremented. When the >> use count hits zero, this means that the VMA is no more used and should be >> freed. >> >> This patch is preparing for 2 kind of VMA access : >> - as usual, under the control of the mmap_sem, >> - without holding the mmap_sem for the speculative page fault handler. >> >> Access done under the control the mmap_sem doesn't require to grab the >> rwlock to protect read access to the mm_rb tree, but access in write must >> be done under the protection of the rwlock too. This affects inserting and >> removing of elements in the RB tree. >> >> The patch is introducing 2 new functions: >> - vma_get() to find a VMA based on an address by holding the new rwlock. >> - vma_put() to release the VMA when its no more used. >> These services are designed to be used when access are made to the RB tree >> without holding the mmap_sem. >> >> When a VMA is removed from the RB tree, its vma->vm_rb field is cleared and >> we rely on the WMB done when releasing the rwlock to serialize the write >> with the RMB done in a later patch to check for the VMA's validity. >> >> When free_vma is called, the file associated with the VMA is closed >> immediately, but the policy and the file structure remained in used until >> the VMA's use count reach 0, which may happens later when exiting an >> in progress speculative page fault. >> >> [1] https://patchwork.kernel.org/patch/5108281/ >> >> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> >> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> >> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxxxxxxx> >> --- >> include/linux/mm.h | 1 + >> include/linux/mm_types.h | 4 ++ >> kernel/fork.c | 3 ++ >> mm/init-mm.c | 3 ++ >> mm/internal.h | 6 +++ >> mm/mmap.c | 115 +++++++++++++++++++++++++++++++++++------------ >> 6 files changed, 104 insertions(+), 28 deletions(-) >> > > [...] > >> diff --git a/mm/mmap.c b/mm/mmap.c >> index 5601f1ef8bb9..a82950960f2e 100644 >> --- a/mm/mmap.c >> +++ b/mm/mmap.c >> @@ -160,6 +160,27 @@ void unlink_file_vma(struct vm_area_struct *vma) >> } >> } >> >> +static void __free_vma(struct vm_area_struct *vma) >> +{ >> + if (vma->vm_file) >> + fput(vma->vm_file); >> + mpol_put(vma_policy(vma)); >> + kmem_cache_free(vm_area_cachep, vma); >> +} >> + >> +#ifdef CONFIG_SPECULATIVE_PAGE_FAULT >> +void put_vma(struct vm_area_struct *vma) >> +{ >> + if (atomic_dec_and_test(&vma->vm_ref_count)) >> + __free_vma(vma); >> +} >> +#else >> +static inline void put_vma(struct vm_area_struct *vma) >> +{ >> + return __free_vma(vma); > > Please drop the "return". Sure ! Thanks. > > Thanks, > Punit > > [...] >
