| Message ID | 20190418135039.19987-1-mkoutny@suse.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | prctl_set_mm: downgrade mmap_sem to read lock | expand |
On Thu, Apr 18, 2019 at 03:50:39PM +0200, Michal Koutný wrote: > I learnt, it's, alas, too late to drop the non PRCTL_SET_MM_MAP calls > [1], so at least downgrade the write acquisition of mmap_sem as in the > patch below (that should be stacked on the previous one or squashed). > > Cyrill, you mentioned lock changes in [1] but the link seems empty. Is > it supposed to be [2]? That could be an alternative to this patch after > some refreshments and clarifications. Yes, seems so. From a glance the patch shold be ok. Michal will review it more carefully today. Thanks!
On Thu 18-04-19 15:50:39, Michal Koutny wrote: > I learnt, it's, alas, too late to drop the non PRCTL_SET_MM_MAP calls > [1], so at least downgrade the write acquisition of mmap_sem as in the > patch below (that should be stacked on the previous one or squashed). > > Cyrill, you mentioned lock changes in [1] but the link seems empty. Is > it supposed to be [2]? That could be an alternative to this patch after > some refreshments and clarifications. > > > [1] https://lore.kernel.org/lkml/20190417165632.GC3040@uranus.lan/ > [2] https://lore.kernel.org/lkml/20180507075606.870903028@gmail.com/ > > ======== > > Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect > arg_start|end and env_start|end in mm_struct") we use arg_lock for > boundaries modifications. Synchronize prctl_set_mm with this lock and > keep mmap_sem for reading only (analogous to what we already do in > prctl_set_mm_map). > > Also, save few cycles by looking up VMA only after performing basic > arguments validation. > > Signed-off-by: Michal Koutný <mkoutny@suse.com> Looks good to me. Please send both patches in one series once you get a review feedback from other people. > --- > kernel/sys.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/kernel/sys.c b/kernel/sys.c > index 12df0e5434b8..bbce0f26d707 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -2125,8 +2125,12 @@ static int prctl_set_mm(int opt, unsigned long addr, > > error = -EINVAL; > > - down_write(&mm->mmap_sem); > - vma = find_vma(mm, addr); > + /* > + * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for > + * a) concurrent sys_brk, b) finding VMA for addr validation. > + */ > + down_read(&mm->mmap_sem); > + spin_lock(&mm->arg_lock); > > prctl_map.start_code = mm->start_code; > prctl_map.end_code = mm->end_code; > @@ -2185,6 +2189,7 @@ static int prctl_set_mm(int opt, unsigned long addr, > if (error) > goto out; > > + vma = find_vma(mm, addr); > switch (opt) { > /* > * If command line arguments and environment > @@ -2218,7 +2223,8 @@ static int prctl_set_mm(int opt, unsigned long addr, > > error = 0; > out: > - up_write(&mm->mmap_sem); > + spin_unlock(&mm->arg_lock); > + up_read(&mm->mmap_sem); > return error; > } > > -- > 2.16.4
Le 18/04/2019 à 15:50, Michal Koutný a écrit : > I learnt, it's, alas, too late to drop the non PRCTL_SET_MM_MAP calls > [1], so at least downgrade the write acquisition of mmap_sem as in the > patch below (that should be stacked on the previous one or squashed). > > Cyrill, you mentioned lock changes in [1] but the link seems empty. Is > it supposed to be [2]? That could be an alternative to this patch after > some refreshments and clarifications. > > > [1] https://lore.kernel.org/lkml/20190417165632.GC3040@uranus.lan/ > [2] https://lore.kernel.org/lkml/20180507075606.870903028@gmail.com/ > > ======== > > Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect > arg_start|end and env_start|end in mm_struct") we use arg_lock for > boundaries modifications. Synchronize prctl_set_mm with this lock and > keep mmap_sem for reading only (analogous to what we already do in > prctl_set_mm_map). > > Also, save few cycles by looking up VMA only after performing basic > arguments validation. > > Signed-off-by: Michal Koutný <mkoutny@suse.com> > --- > kernel/sys.c | 12 +++++++++--- > 1 file changed, 9 insertions(+), 3 deletions(-) > > diff --git a/kernel/sys.c b/kernel/sys.c > index 12df0e5434b8..bbce0f26d707 100644 > --- a/kernel/sys.c > +++ b/kernel/sys.c > @@ -2125,8 +2125,12 @@ static int prctl_set_mm(int opt, unsigned long addr, > > error = -EINVAL; > > - down_write(&mm->mmap_sem); > - vma = find_vma(mm, addr); > + /* > + * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for > + * a) concurrent sys_brk, b) finding VMA for addr validation. > + */ > + down_read(&mm->mmap_sem); > + spin_lock(&mm->arg_lock); > > prctl_map.start_code = mm->start_code; > prctl_map.end_code = mm->end_code; > @@ -2185,6 +2189,7 @@ static int prctl_set_mm(int opt, unsigned long addr, > if (error) > goto out; > > + vma = find_vma(mm, addr); Why is find_vma() called while holding the arg_lock ? To limit the time the spinlock is held, would it be better to read_lock(mmap_sem) find_vma() spin_lock(arg_lock) .. out: spin_unlock() up_read(mmap_sem) Not sure this would change a lot the performance anyway. > switch (opt) { > /* > * If command line arguments and environment > @@ -2218,7 +2223,8 @@ static int prctl_set_mm(int opt, unsigned long addr, > > error = 0; > out: > - up_write(&mm->mmap_sem); > + spin_unlock(&mm->arg_lock); > + up_read(&mm->mmap_sem); > return error; > } > >
On Thu, Apr 18, 2019 at 03:50:39PM +0200, Michal Koutný wrote: > I learnt, it's, alas, too late to drop the non PRCTL_SET_MM_MAP calls > [1], so at least downgrade the write acquisition of mmap_sem as in the > patch below (that should be stacked on the previous one or squashed). > > Cyrill, you mentioned lock changes in [1] but the link seems empty. Is > it supposed to be [2]? That could be an alternative to this patch after > some refreshments and clarifications. > > > [1] https://lore.kernel.org/lkml/20190417165632.GC3040@uranus.lan/ > [2] https://lore.kernel.org/lkml/20180507075606.870903028@gmail.com/ > > ======== > > Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect > arg_start|end and env_start|end in mm_struct") we use arg_lock for > boundaries modifications. Synchronize prctl_set_mm with this lock and > keep mmap_sem for reading only (analogous to what we already do in > prctl_set_mm_map). > > Also, save few cycles by looking up VMA only after performing basic > arguments validation. > > Signed-off-by: Michal Koutný <mkoutny@suse.com> Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com> As Laurent mentioned we might move vma lookup before the spinlock, but this might be done on top of the series.
========
Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect
arg_start|end and env_start|end in mm_struct") we use arg_lock for
boundaries modifications. Synchronize prctl_set_mm with this lock and
keep mmap_sem for reading only (analogous to what we already do in
prctl_set_mm_map).
Also, save few cycles by looking up VMA only after performing basic
arguments validation.
Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
kernel/sys.c | 12 +++++++++---
1 file changed, 9 insertions(+), 3 deletions(-)
diff --git a/kernel/sys.c b/kernel/sys.c
index 12df0e5434b8..bbce0f26d707 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2125,8 +2125,12 @@ static int prctl_set_mm(int opt, unsigned long addr,
error = -EINVAL;
- down_write(&mm->mmap_sem);
- vma = find_vma(mm, addr);
+ /*
+ * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for
+ * a) concurrent sys_brk, b) finding VMA for addr validation.
+ */
+ down_read(&mm->mmap_sem);
+ spin_lock(&mm->arg_lock);
prctl_map.start_code = mm->start_code;
prctl_map.end_code = mm->end_code;
@@ -2185,6 +2189,7 @@ static int prctl_set_mm(int opt, unsigned long addr,
if (error)
goto out;
+ vma = find_vma(mm, addr);
switch (opt) {
/*
* If command line arguments and environment
@@ -2218,7 +2223,8 @@ static int prctl_set_mm(int opt, unsigned long addr,
error = 0;
out:
- up_write(&mm->mmap_sem);
+ spin_unlock(&mm->arg_lock);
+ up_read(&mm->mmap_sem);
return error;
}