Re: [4.15-rc9] fs_reclaim lockdep trace
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: 2018-02-01 11:37:29
Also in:
linux-btrfs, lkml, netdev
Peter Zijlstra wrote:
On Mon, Jan 29, 2018 at 08:47:20PM +0900, Tetsuo Handa wrote:quoted
Peter Zijlstra wrote:quoted
On Sun, Jan 28, 2018 at 02:55:28PM +0900, Tetsuo Handa wrote:quoted
This warning seems to be caused by commit d92a8cfcb37ecd13 ("locking/lockdep: Rework FS_RECLAIM annotation") which moved the location of /* this guy won't enter reclaim */ if ((current->flags & PF_MEMALLOC) && !(gfp_mask & __GFP_NOMEMALLOC)) return false; check added by commit cf40bd16fdad42c0 ("lockdep: annotate reclaim context (__GFP_NOFS)").I'm not entirly sure I get what you mean here. How did I move it? It was part of lockdep_trace_alloc(), if __GFP_NOMEMALLOC was set, it would not mark the lock as held.d92a8cfcb37ecd13 replaced lockdep_set_current_reclaim_state() with fs_reclaim_acquire(), and removed current->lockdep_recursion handling. ---------- # git show d92a8cfcb37ecd13 | grep recursion -# define INIT_LOCKDEP .lockdep_recursion = 0, .lockdep_reclaim_gfp = 0, +# define INIT_LOCKDEP .lockdep_recursion = 0, unsigned int lockdep_recursion; - if (unlikely(current->lockdep_recursion)) - current->lockdep_recursion = 1; - current->lockdep_recursion = 0; - * context checking code. This tests GFP_FS recursion (a lock taken ----------That should not matter at all. The only case that would matter for is if lockdep itself would ever call into lockdep again. Not something that happens here.quoted
quoted
The new code has it in fs_reclaim_acquire/release to the same effect, if __GFP_NOMEMALLOC, we'll not acquire/release the lock.Excuse me, but I can't catch. We currently acquire/release __fs_reclaim_map if __GFP_NOMEMALLOC.Right, got the case inverted, same difference though. Before we'd do mark_held_lock(), now we do acquire/release under the same conditions.quoted
quoted
quoted
Since __kmalloc_reserve() from __alloc_skb() adds __GFP_NOMEMALLOC | __GFP_NOWARN to gfp_mask, __need_fs_reclaim() is failing to return false despite PF_MEMALLOC context (and resulted in lockdep warning).But that's correct right, __GFP_NOMEMALLOC should negate PF_MEMALLOC. That's what the name says.__GFP_NOMEMALLOC negates PF_MEMALLOC regarding what watermark that allocation request should use.Right.quoted
But at the same time, PF_MEMALLOC negates __GFP_DIRECT_RECLAIM.Ah indeed.quoted
Then, how can fs_reclaim contribute to deadlock?Not sure it can. But if we're going to allow this, it needs to come with a clear description on why. Not a few clues to a puzzle.
Let's decode Dave's report.
----------
stack backtrace:
CPU: 3 PID: 24800 Comm: sshd Not tainted 4.15.0-rc9-backup-debug+ #1
Call Trace:
dump_stack+0xbc/0x13f
__lock_acquire+0xa09/0x2040
lock_acquire+0x12e/0x350
fs_reclaim_acquire.part.102+0x29/0x30
kmem_cache_alloc+0x3d/0x2c0
alloc_extent_state+0xa7/0x410
__clear_extent_bit+0x3ea/0x570
try_release_extent_mapping+0x21a/0x260
__btrfs_releasepage+0xb0/0x1c0
btrfs_releasepage+0x161/0x170
try_to_release_page+0x162/0x1c0
shrink_page_list+0x1d5a/0x2fb0
shrink_inactive_list+0x451/0x940
shrink_node_memcg.constprop.88+0x4c9/0x5e0
shrink_node+0x12d/0x260
try_to_free_pages+0x418/0xaf0
__alloc_pages_slowpath+0x976/0x1790
__alloc_pages_nodemask+0x52c/0x5c0
new_slab+0x374/0x3f0
___slab_alloc.constprop.81+0x47e/0x5a0
__slab_alloc.constprop.80+0x32/0x60
__kmalloc_track_caller+0x267/0x310
__kmalloc_reserve.isra.40+0x29/0x80
__alloc_skb+0xee/0x390
sk_stream_alloc_skb+0xb8/0x340
----------
struct sk_buff *sk_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp, bool force_schedule) {
skb = alloc_skb_fclone(size + sk->sk_prot->max_header, gfp) = { // gfp == GFP_KERNEL
static inline struct sk_buff *alloc_skb_fclone(unsigned int size, gfp_t priority) { // priority == GFP_KERNEL
return __alloc_skb(size, priority, SKB_ALLOC_FCLONE, NUMA_NO_NODE) = {
data = kmalloc_reserve(size, gfp_mask, node, &pfmemalloc) = { // gfp_mask == GFP_KERNEL
obj = kmalloc_node_track_caller(size, flags | __GFP_NOMEMALLOC | __GFP_NOWARN, node) = { // flags == GFP_KERNEL
__kmalloc_node_track_caller(size, GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN, node) = {
void *__kmalloc_node_track_caller(size_t size, gfp_t gfpflags, int node, unsigned long caller) { // gfpflags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
ret = slab_alloc_node(s, gfpflags, node, caller) = { // gfpflags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
static __always_inline void *slab_alloc_node(struct kmem_cache *s, gfp_t gfpflags, int node, unsigned long addr) { // gfpflags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
s = slab_pre_alloc_hook(s, gfpflags) = { // gfpflags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s, gfp_t flags) { // gfpflags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
fs_reclaim_acquire(flags) = { // flags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
void fs_reclaim_acquire(gfp_t gfp_mask) { // gfp_mask == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
if (__need_fs_reclaim(gfp_mask)) // true due to gfp_mask == GFP_KERNEL | __GFP_NOMEMALLOC
lock_map_acquire(&__fs_reclaim_map); // acquires __fs_reclaim_map
}
}
}
fs_reclaim_release(flags); // releases __fs_reclaim_map
}
object = __slab_alloc(s, gfpflags, node, addr, c) = { // gfpflags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
p = ___slab_alloc(s, gfpflags, node, addr, c) = { // gfpflags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
freelist = new_slab_objects(s, gfpflags, node, &c) = {
page = new_slab(s, flags, node) = { // flags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
return allocate_slab(s, flags & (GFP_RECLAIM_MASK | GFP_CONSTRAINT_MASK), node) = {
page = alloc_slab_page(s, alloc_gfp, node, oo) = { // alloc_gfp == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
page = alloc_pages(flags, order) { // flags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
return alloc_pages_current(gfp_mask, order) = { //gfp_mask == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
page = __alloc_pages_nodemask(gfp, order, policy_node(gfp, pol, numa_node_id()), policy_nodemask(gfp, pol)) = { // gfp == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
page = __alloc_pages_slowpath(alloc_mask, order, &ac) = { // alloc_mask == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac, &did_some_progress) = { // gfp_mask == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
*did_some_progress = __perform_reclaim(gfp_mask, order, ac) = { // gfp_mask == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
noreclaim_flag = memalloc_noreclaim_save(); // Sets PF_MEMALLOC
fs_reclaim_acquire(flags) = { // flags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
void fs_reclaim_acquire(gfp_t gfp_mask) { // gfp_mask == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
if (__need_fs_reclaim(gfp_mask)) // true due to gfp_mask == GFP_KERNEL | __GFP_NOMEMALLOC
lock_map_acquire(&__fs_reclaim_map); // acquires __fs_reclaim_map
}
}
progress = try_to_free_pages(ac->zonelist, order, gfp_mask, ac->nodemask) = {
nr_reclaimed = do_try_to_free_pages(zonelist, &sc) = {
shrink_zones(zonelist, sc) = {
shrink_node(zone->zone_pgdat, sc) = {
shrink_node_memcg(pgdat, memcg, sc, &lru_pages) = {
nr_reclaimed += shrink_list(lru, nr_to_scan, lruvec, memcg, sc) = {
return shrink_inactive_list(nr_to_scan, lruvec, sc, lru) = {
nr_reclaimed = shrink_page_list(&page_list, pgdat, sc, 0, &stat, false) = {
if (!try_to_release_page(page, sc->gfp_mask))
goto activate_locked = {
return mapping->a_ops->releasepage(page, gfp_mask) = {
static int btrfs_releasepage(struct page *page, gfp_t gfp_flags) { // gfp_flags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
return __btrfs_releasepage(page, gfp_flags) = {
ret = try_release_extent_mapping(map, tree, page, gfp_flags) = {
return try_release_extent_state(map, tree, page, mask) = { // mask == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
ret = clear_extent_bit(tree, start, end, ~(EXTENT_LOCKED | EXTENT_NODATASUM), 0, 0, NULL, mask) = {
return __clear_extent_bit(tree, start, end, bits, wake, delete, cached, mask, NULL) = {
prealloc = alloc_extent_state(mask) = {
state = kmem_cache_alloc(extent_state_cache, mask) = {
void *ret = slab_alloc(s, gfpflags, _RET_IP_) = { // gfpflags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
return slab_alloc_node(s, gfpflags, NUMA_NO_NODE, addr) = {
s = slab_pre_alloc_hook(s, gfpflags) = {
static inline struct kmem_cache *slab_pre_alloc_hook(struct kmem_cache *s, gfp_t flags) { // gfpflags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
fs_reclaim_acquire(flags) = { // flags == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
void fs_reclaim_acquire(gfp_t gfp_mask) { // gfp_mask == GFP_KERNEL | __GFP_NOMEMALLOC | __GFP_NOWARN
if (__need_fs_reclaim(gfp_mask)) // true due to gfp_mask == GFP_KERNEL | __GFP_NOMEMALLOC despite PF_MEMALLOC
lock_map_acquire(&__fs_reclaim_map); // acquires __fs_reclaim_map nestedly and lockdep complains
}
}
}
fs_reclaim_release(flags); // releases __fs_reclaim_map
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
}
That is, all reclaim code is simply propagating __GFP_NOMEMALLOC added by kmalloc_reserve(), and
despite memory allocation from try_to_free_pages() path won't do direct reclaim due to PF_MEMALLOC,
fs_reclaim_acquire() from slab_pre_alloc_hook() from try_to_free_pages() path is failing to find that
this allocation will not do direct reclaim due to PF_MEMALLOC (due to
/* this guy won't enter reclaim */
if ((current->flags & PF_MEMALLOC) && !(gfp_mask & __GFP_NOMEMALLOC))
return false;
check in __need_fs_reclaim()).
After all, nested GFP_FS allocations cannot occur (whatever GFP flags are passed)
because such allocation will not do direct reclaim due to PF_MEMALLOC.
Now, even if its not strictly a deadlock, there is something to be said for flagging GFP_FS allocs that lead to nested GFP_FS allocs, do we ever want to allow that?
Since PF_MEMALLOC negates __GFP_DIRECT_RECLAIM, propagating unmodified GFP flags (like above) is safe as long as dependency is within current thread. So, how to fix this? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>