From: Hyeonggon Yoo <42.hyeyoo@xxxxxxxxx> To: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: linux-mm@xxxxxxxxx, liam.howlett@xxxxxxxxxx, surenb@xxxxxxxxxx, ldufour@xxxxxxxxxxxxx, michel@xxxxxxxxxxxxxx, vbabka@xxxxxxx, linux-kernel@xxxxxxxxxxxxxxx Bcc: Subject: Re: [QUESTION] about the maple tree and current status of mmap_lock scalability Reply-To: In-Reply-To: <Y63FmaNoLAcdsLaU@xxxxxxxxxxxxxxxxxxxx> On Thu, Dec 29, 2022 at 04:51:37PM +0000, Matthew Wilcox wrote: > On Thu, Dec 29, 2022 at 11:22:28PM +0900, Hyeonggon Yoo wrote: > > On Wed, Dec 28, 2022 at 08:50:36PM +0000, Matthew Wilcox wrote: > > > The long term goal is even larger than this. Ideally, the VMA tree > > > would be protected by a spinlock rather than a mutex. > > > > You mean replacing mmap_lock rwsem with a spinlock? > > How is that possible if readers can take it for page fault? > > The mmap_lock is taken for many, many things. So the plan was to > have a spinlock in the maple tree (indeed, there's still one there; > it's just in a union with the lockdep_map_p). VMA readers would walk > the tree protected only by RCU; VMA writers would take the spinlock > while modifying the tree. The work Suren, Liam & I are engaged in > still uses the mmap semaphore for writers, but we do walk the tree > under RCU protection. > Thanks, I get it. so it's for less overhead for maple tree modification. > > > While I've read the RCUVM paper, I wouldn't say it was particularly an > > > inspiration. The Maple Tree is independent of the VM; it's a general > > > purpose B-tree. > > > > My intention was to ask how to synchronize with other VMA operations > > after the tree traversal with RCU. (Because it's unreasonable to handle > > page fault in RCU read-side critical section) > > > > Per-VMA lock seem to solve it by taking the VMA lock in read mode within > > RCU read-side critical section. > > Right, but it's a little more complex than that. The real "lock" on > the VMA is actually a sequence count. https://lwn.net/Articles/906852/ > does a good job of explaining it, but the VMA lock is really there as > a convenient way for the writer to wait for readers to be sufficiently > "finished" with handling the page fault that any conflicting changes > will be correctly retired. Oh, thanks, nice article! > https://www.infradead.org/~willy/linux/store-free-page-faults.html > outlines how I intend to proceed from Suren's current scheme (where > RCU is only used to protect the tree walk) to using RCU for the > entire page fault. Thank you for sharing this your outlines. Okay, so the planned scheme is: 1. Try to process entire page fault under RCU protection - if failed, goto 2. if succeeded, goto 4. 2. Fall back to Suren's scheme (try to take VMA rwsem) - if failed, goto 3. if succeeded, goto 4. 3. Fall back to mmap_lock - goto 4. 4. Finish page fault. To implement 1, __p*d_alloc() need to take gfp flags not to sleep in RCU read-side critical section. What about introducing PF_MEMALLOC_NOWAIT process flag forcing GFP_NOWAIT | __GFP_NOWARN similar to PF_MEMALLOC_NO{FS,IO}, looking like this? Will be less churn. diff --git a/include/linux/sched.h b/include/linux/sched.h index 853d08f7562b..77b88f30523b 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1725,7 +1725,7 @@ extern struct pid *cad_pid; #define PF_USED_MATH 0x00002000 /* If unset the fpu must be initialized before use */ #define PF__HOLE__00004000 0x00004000 #define PF_NOFREEZE 0x00008000 /* This thread should not be frozen */ -#define PF__HOLE__00010000 0x00010000 +#define PF_MEMALLOC_NOWAIT 0x00010000 /* All allocation requests will force GFP_NOWAIT | __GFP_NOWARN */ #define PF_KSWAPD 0x00020000 /* I am kswapd */ #define PF_MEMALLOC_NOFS 0x00040000 /* All allocation requests will inherit GFP_NOFS */ #define PF_MEMALLOC_NOIO 0x00080000 /* All allocation requests will inherit GFP_NOIO */ diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h index 2a243616f222..4a1196646951 100644 --- a/include/linux/sched/mm.h +++ b/include/linux/sched/mm.h @@ -204,7 +204,8 @@ static inline gfp_t current_gfp_context(gfp_t flags) { unsigned int pflags = READ_ONCE(current->flags); - if (unlikely(pflags & (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS | PF_MEMALLOC_PIN))) { + if (unlikely(pflags & (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS + | PF_MEMALLOC_PIN | PF_MEMALLOC_NOWAIT))) { /* * NOIO implies both NOIO and NOFS and it is a weaker context * so always make sure it makes precedence @@ -216,6 +217,8 @@ static inline gfp_t current_gfp_context(gfp_t flags) if (pflags & PF_MEMALLOC_PIN) flags &= ~__GFP_MOVABLE; + if (pflags & PF_MEMALLOC_NOWAIT) + flags = GFP_NOWAIT | __GFP_NOWARN; } return flags; } @@ -305,6 +308,18 @@ static inline void memalloc_noio_restore(unsigned int flags) current->flags = (current->flags & ~PF_MEMALLOC_NOIO) | flags; } +static inline unsigned int memalloc_nowait_save(void) +{ + unsigned int flags = current->flags & PF_MEMALLOC_NOWAIT; + current->flags |= PF_MEMALLOC_NOWAIT; + return flags; +} + +static inline void memalloc_nowait_restore(unsigned int flags) +{ + current->flags = (current->flags & ~PF_MEMALLOC_NOWAIT) | flags; -- Thanks, Hyeonggon
