On 03/04/2018 02:11, David Rientjes wrote: > On Tue, 13 Mar 2018, Laurent Dufour 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> > > Can __free_vma() be generalized for mm/nommu.c's delete_vma() and > do_mmap()? Good question ! I guess if there is no mmu, there is no page fault, so no speculative page fault and this patch is clearly required by the speculative page fault handler. By the I should probably make CONFIG_SPECULATIVE_PAGE_FAULT dependent on CONFIG_MMU. This being said, if your idea is to extend the mm_rb tree rwlocking to the nommu case, then this is another story, and I wondering if there is a real need in such case. But I've to admit I'm not so familliar with kernel built for mmuless systems. Am I missing something ? Thanks, Laurent.
