On 11/26/2014 06:35 PM, Michal Hocko wrote: > On Tue 25-11-14 16:00:06, Michal Hocko wrote: >> On Tue 25-11-14 16:13:16, Konstantin Khlebnikov wrote: >>> On Tue, Nov 25, 2014 at 1:59 PM, Michal Hocko <mhocko@xxxxxxx> wrote: >>>> On Mon 24-11-14 11:09:40, Konstantin Khlebnikov wrote: >>>>> On Thu, Nov 20, 2014 at 6:03 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote: >>>>>> On Thu, Nov 20, 2014 at 5:50 PM, Rik van Riel <riel@xxxxxxxxxx> wrote: >>>>>>> -----BEGIN PGP SIGNED MESSAGE----- >>>>>>> Hash: SHA1 >>>>>>> >>>>>>> On 11/20/2014 09:42 AM, Konstantin Khlebnikov wrote: >>>>>>> >>>>>>>> I'm thinking about limitation for reusing anon_vmas which might >>>>>>>> increase performance without breaking asymptotic estimation of >>>>>>>> count anon_vma in the worst case. For example this heuristic: allow >>>>>>>> to reuse only anon_vma with single direct descendant. It seems >>>>>>>> there will be arount up to two times more anon_vmas but >>>>>>>> false-aliasing must be much lower. >>>>> >>>>> Done. RFC patch in attachment. > > Ok, finally managed to untagnle myself from vma chains and your patch > makes sense to me, it is quite clever actually. Here is it including the > fixup. > --- >> From 1d4b0b38198c69ecfeb37670cb1dda767a802c9a Mon Sep 17 00:00:00 2001 >> From: Konstantin Khlebnikov <koct9i@xxxxxxxxx> >> Date: Tue, 25 Nov 2014 10:54:44 +0100 >> Subject: [PATCH] mm: prevent endless growth of anon_vma hierarchy >> >> Constantly forking task causes unlimited grow of anon_vma chain. >> Each next child allocate new level of anon_vmas and links vmas to all >> previous levels because it inherits pages from them. None of anon_vmas >> cannot be freed because there might be pages which points to them. >> >> This patch adds heuristic which decides to reuse existing anon_vma instead >> of forking new one. It counts vmas and direct descendants for each anon_vma. >> Anon_vma with degree lower than two will be reused at next fork. >> As a result each anon_vma has either alive vma or at least two descendants, >> endless chains are no longer possible and count of anon_vmas is no more than >> two times more than count of vmas. >> >> Signed-off-by: Konstantin Khlebnikov <koct9i@xxxxxxxxx> >> Link: http://lkml.kernel.org/r/20120816024610.GA5350@xxxxxxxxxxxxxxxxxxxxxxx > > Tested-by: Michal Hocko <mhocko@xxxxxxx> > Reviewed-by: Michal Hocko <mhocko@xxxxxxx> > > and I guess > Reported-by: Daniel Forrest <dan.forrest@xxxxxxxxxxxxx> Tested-by: Jerome Marchand <jmarchan@xxxxxxxxxx> Minor nitpicks below. > > who somehow vanished from CC list (added back) would be appropriate as > well. > > plus > > Fixes: 5beb49305251 (mm: change anon_vma linking to fix multi-process server scalability issue) > and mark it for stable > > Thanks! > >> --- >> include/linux/rmap.h | 16 ++++++++++++++++ >> mm/rmap.c | 29 ++++++++++++++++++++++++++++- >> 2 files changed, 44 insertions(+), 1 deletion(-) >> >> diff --git a/include/linux/rmap.h b/include/linux/rmap.h >> index c0c2bce6b0b7..b1d140c20b37 100644 >> --- a/include/linux/rmap.h >> +++ b/include/linux/rmap.h >> @@ -45,6 +45,22 @@ struct anon_vma { >> * mm_take_all_locks() (mm_all_locks_mutex). >> */ >> struct rb_root rb_root; /* Interval tree of private "related" vmas */ >> + >> + /* >> + * Count of child anon_vmas and VMAs which points to this anon_vma. >> + * >> + * This counter is used for making decision about reusing old anon_vma >> + * instead of forking new one. It allows to detect anon_vmas which have >> + * just one direct descendant and no vmas. Reusing such anon_vma not >> + * leads to significant preformance regression but prevents degradation Does it or does it not lead to significant performance issue? I can't tell. >> + * of anon_vma hierarchy to endless linear chain. >> + * >> + * Root anon_vma is never reused because it is its own parent and it has >> + * at leat one vma or child, thus at fork it's degree is at least 2. s/leat/least/ Thanks, Jerome >> + */ >> + unsigned degree; >> + >> + struct anon_vma *parent; /* Parent of this anon_vma */ >> }; >> >> /* >> diff --git a/mm/rmap.c b/mm/rmap.c >> index 19886fb2f13a..40ae8184a1e1 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -72,6 +72,8 @@ static inline struct anon_vma *anon_vma_alloc(void) >> anon_vma = kmem_cache_alloc(anon_vma_cachep, GFP_KERNEL); >> if (anon_vma) { >> atomic_set(&anon_vma->refcount, 1); >> + anon_vma->degree = 1; /* Reference for first vma */ >> + anon_vma->parent = anon_vma; >> /* >> * Initialise the anon_vma root to point to itself. If called >> * from fork, the root will be reset to the parents anon_vma. >> @@ -188,6 +190,7 @@ int anon_vma_prepare(struct vm_area_struct *vma) >> if (likely(!vma->anon_vma)) { >> vma->anon_vma = anon_vma; >> anon_vma_chain_link(vma, avc, anon_vma); >> + anon_vma->degree++; >> allocated = NULL; >> avc = NULL; >> } >> @@ -256,7 +259,17 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src) >> anon_vma = pavc->anon_vma; >> root = lock_anon_vma_root(root, anon_vma); >> anon_vma_chain_link(dst, avc, anon_vma); >> + >> + /* >> + * Reuse existing anon_vma if its degree lower than two, >> + * that means it has no vma and just one anon_vma child. >> + */ >> + if (!dst->anon_vma && anon_vma != src->anon_vma && >> + anon_vma->degree < 2) >> + dst->anon_vma = anon_vma; >> } >> + if (dst->anon_vma) >> + dst->anon_vma->degree++; >> unlock_anon_vma_root(root); >> return 0; >> >> @@ -279,6 +292,9 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) >> if (!pvma->anon_vma) >> return 0; >> >> + /* Drop inherited anon_vma, we'll reuse old one or allocate new. */ >> + vma->anon_vma = NULL; >> + >> /* >> * First, attach the new VMA to the parent VMA's anon_vmas, >> * so rmap can find non-COWed pages in child processes. >> @@ -286,6 +302,10 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) >> if (anon_vma_clone(vma, pvma)) >> return -ENOMEM; >> >> + /* An old anon_vma has been reused. */ >> + if (vma->anon_vma) >> + return 0; >> + >> /* Then add our own anon_vma. */ >> anon_vma = anon_vma_alloc(); >> if (!anon_vma) >> @@ -299,6 +319,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) >> * lock any of the anon_vmas in this anon_vma tree. >> */ >> anon_vma->root = pvma->anon_vma->root; >> + anon_vma->parent = pvma->anon_vma; >> /* >> * With refcounts, an anon_vma can stay around longer than the >> * process it belongs to. The root anon_vma needs to be pinned until >> @@ -309,6 +330,7 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma) >> vma->anon_vma = anon_vma; >> anon_vma_lock_write(anon_vma); >> anon_vma_chain_link(vma, avc, anon_vma); >> + anon_vma->parent->degree++; >> anon_vma_unlock_write(anon_vma); >> >> return 0; >> @@ -339,12 +361,16 @@ void unlink_anon_vmas(struct vm_area_struct *vma) >> * Leave empty anon_vmas on the list - we'll need >> * to free them outside the lock. >> */ >> - if (RB_EMPTY_ROOT(&anon_vma->rb_root)) >> + if (RB_EMPTY_ROOT(&anon_vma->rb_root)) { >> + anon_vma->parent->degree--; >> continue; >> + } >> >> list_del(&avc->same_vma); >> anon_vma_chain_free(avc); >> } >> + if (vma->anon_vma) >> + vma->anon_vma->degree--; >> unlock_anon_vma_root(root); >> >> /* >> @@ -355,6 +381,7 @@ void unlink_anon_vmas(struct vm_area_struct *vma) >> list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) { >> struct anon_vma *anon_vma = avc->anon_vma; >> >> + BUG_ON(anon_vma->degree); >> put_anon_vma(anon_vma); >> >> list_del(&avc->same_vma); >> -- >> 2.1.3 >
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