__SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
lock the page. Following zsmalloc.c's example we call trylock_page() and
unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
passed in locked, as documentation.
Signed-off-by: Henry Burns <[email protected]>
Suggested-by: Vitaly Wool <[email protected]>
---
Changelog since v1:
- Added an if statement around WARN_ON(trylock_page(page)) to avoid
unlocking a page locked by a someone else.
mm/z3fold.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/mm/z3fold.c b/mm/z3fold.c
index e174d1549734..6341435b9610 100644
--- a/mm/z3fold.c
+++ b/mm/z3fold.c
@@ -918,7 +918,10 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
set_bit(PAGE_HEADLESS, &page->private);
goto headless;
}
- __SetPageMovable(page, pool->inode->i_mapping);
+ if (!WARN_ON(!trylock_page(page))) {
+ __SetPageMovable(page, pool->inode->i_mapping);
+ unlock_page(page);
+ }
z3fold_page_lock(zhdr);
found:
@@ -1325,6 +1328,7 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
VM_BUG_ON_PAGE(!PageMovable(page), page);
VM_BUG_ON_PAGE(!PageIsolated(page), page);
+ VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
zhdr = page_address(page);
pool = zhdr_to_pool(zhdr);
--
2.22.0.410.gd8fdbe21b5-goog
On Mon, Jul 1, 2019 at 5:51 PM Henry Burns <[email protected]> wrote:
>
> __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
> lock the page. Following zsmalloc.c's example we call trylock_page() and
> unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
> passed in locked, as documentation.
>
> Signed-off-by: Henry Burns <[email protected]>
> Suggested-by: Vitaly Wool <[email protected]>
> ---
> Changelog since v1:
> - Added an if statement around WARN_ON(trylock_page(page)) to avoid
> unlocking a page locked by a someone else.
>
> mm/z3fold.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index e174d1549734..6341435b9610 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -918,7 +918,10 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
> set_bit(PAGE_HEADLESS, &page->private);
> goto headless;
> }
> - __SetPageMovable(page, pool->inode->i_mapping);
> + if (!WARN_ON(!trylock_page(page))) {
> + __SetPageMovable(page, pool->inode->i_mapping);
> + unlock_page(page);
> + }
Can you please comment why lock_page() is not used here?
> z3fold_page_lock(zhdr);
>
> found:
> @@ -1325,6 +1328,7 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
>
> VM_BUG_ON_PAGE(!PageMovable(page), page);
> VM_BUG_ON_PAGE(!PageIsolated(page), page);
> + VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
>
> zhdr = page_address(page);
> pool = zhdr_to_pool(zhdr);
> --
> 2.22.0.410.gd8fdbe21b5-goog
>
On Mon, Jul 1, 2019 at 6:00 PM Shakeel Butt <[email protected]> wrote:
>
> On Mon, Jul 1, 2019 at 5:51 PM Henry Burns <[email protected]> wrote:
> >
> > __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
> > lock the page. Following zsmalloc.c's example we call trylock_page() and
> > unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
> > passed in locked, as documentation.
> >
> > Signed-off-by: Henry Burns <[email protected]>
> > Suggested-by: Vitaly Wool <[email protected]>
> > ---
> > Changelog since v1:
> > - Added an if statement around WARN_ON(trylock_page(page)) to avoid
> > unlocking a page locked by a someone else.
> >
> > mm/z3fold.c | 6 +++++-
> > 1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/z3fold.c b/mm/z3fold.c
> > index e174d1549734..6341435b9610 100644
> > --- a/mm/z3fold.c
> > +++ b/mm/z3fold.c
> > @@ -918,7 +918,10 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
> > set_bit(PAGE_HEADLESS, &page->private);
> > goto headless;
> > }
> > - __SetPageMovable(page, pool->inode->i_mapping);
> > + if (!WARN_ON(!trylock_page(page))) {
> > + __SetPageMovable(page, pool->inode->i_mapping);
> > + unlock_page(page);
> > + }
>
> Can you please comment why lock_page() is not used here?
Since z3fold_alloc can be called in atomic or non atomic context,
calling lock_page() could trigger a number of
warnings about might_sleep() being called in atomic context. WARN_ON
should avoid the problem described
above as well, and in any weird condition where someone else has the
page lock, we can avoid calling
__SetPageMovable().
>
> > z3fold_page_lock(zhdr);
> >
> > found:
> > @@ -1325,6 +1328,7 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
> >
> > VM_BUG_ON_PAGE(!PageMovable(page), page);
> > VM_BUG_ON_PAGE(!PageIsolated(page), page);
> > + VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
> >
> > zhdr = page_address(page);
> > pool = zhdr_to_pool(zhdr);
> > --
> > 2.22.0.410.gd8fdbe21b5-goog
> >
On Tue, Jul 2, 2019 at 3:51 AM Henry Burns <[email protected]> wrote:
>
> __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
> lock the page. Following zsmalloc.c's example we call trylock_page() and
> unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
> passed in locked, as documentation.
>
> Signed-off-by: Henry Burns <[email protected]>
> Suggested-by: Vitaly Wool <[email protected]>
Acked-by: Vitaly Wool <[email protected]>
Thanks!
> ---
> Changelog since v1:
> - Added an if statement around WARN_ON(trylock_page(page)) to avoid
> unlocking a page locked by a someone else.
>
> mm/z3fold.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index e174d1549734..6341435b9610 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -918,7 +918,10 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
> set_bit(PAGE_HEADLESS, &page->private);
> goto headless;
> }
> - __SetPageMovable(page, pool->inode->i_mapping);
> + if (!WARN_ON(!trylock_page(page))) {
> + __SetPageMovable(page, pool->inode->i_mapping);
> + unlock_page(page);
> + }
> z3fold_page_lock(zhdr);
>
> found:
> @@ -1325,6 +1328,7 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
>
> VM_BUG_ON_PAGE(!PageMovable(page), page);
> VM_BUG_ON_PAGE(!PageIsolated(page), page);
> + VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
>
> zhdr = page_address(page);
> pool = zhdr_to_pool(zhdr);
> --
> 2.22.0.410.gd8fdbe21b5-goog
>
On Mon, 1 Jul 2019, Henry Burns wrote:
> __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
> lock the page. Following zsmalloc.c's example we call trylock_page() and
> unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
> passed in locked, as documentation.
>
> Signed-off-by: Henry Burns <[email protected]>
> Suggested-by: Vitaly Wool <[email protected]>
Acked-by: David Rientjes <[email protected]>
On Mon, 1 Jul 2019 18:16:30 -0700 Henry Burns <[email protected]> wrote:
> Cc: Vitaly Wool <[email protected]>, Vitaly Vul <[email protected]>
Are these the same person?
> Subject: Re: [PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable()
> Date: Mon, 1 Jul 2019 18:16:30 -0700
>
> On Mon, Jul 1, 2019 at 6:00 PM Shakeel Butt <[email protected]> wrote:
> >
> > On Mon, Jul 1, 2019 at 5:51 PM Henry Burns <[email protected]> wrote:
> > >
> > > __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
> > > lock the page. Following zsmalloc.c's example we call trylock_page() and
> > > unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
> > > passed in locked, as documentation.
The changelog still doesn't mention that this bug triggers a
VM_BUG_ON_PAGE(). It should do so. I did this:
: __SetPageMovable() expects its page to be locked, but z3fold.c doesn't
: lock the page. This triggers the VM_BUG_ON_PAGE(!PageLocked(page), page)
: in __SetPageMovable().
:
: Following zsmalloc.c's example we call trylock_page() and unlock_page().
: Also make z3fold_page_migrate() assert that newpage is passed in locked,
: as per the documentation.
I'll add a cc:stable to this fix.
> > > Signed-off-by: Henry Burns <[email protected]>
> > > Suggested-by: Vitaly Wool <[email protected]>
> > > ---
> > > Changelog since v1:
> > > - Added an if statement around WARN_ON(trylock_page(page)) to avoid
> > > unlocking a page locked by a someone else.
> > >
> > > mm/z3fold.c | 6 +++++-
> > > 1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/mm/z3fold.c b/mm/z3fold.c
> > > index e174d1549734..6341435b9610 100644
> > > --- a/mm/z3fold.c
> > > +++ b/mm/z3fold.c
> > > @@ -918,7 +918,10 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
> > > set_bit(PAGE_HEADLESS, &page->private);
> > > goto headless;
> > > }
> > > - __SetPageMovable(page, pool->inode->i_mapping);
> > > + if (!WARN_ON(!trylock_page(page))) {
> > > + __SetPageMovable(page, pool->inode->i_mapping);
> > > + unlock_page(page);
> > > + }
> >
> > Can you please comment why lock_page() is not used here?
Shakeel asked "please comment" (ie, please add a code comment), not
"please comment on". Subtle ;)
> Since z3fold_alloc can be called in atomic or non atomic context,
> calling lock_page() could trigger a number of
> warnings about might_sleep() being called in atomic context. WARN_ON
> should avoid the problem described
> above as well, and in any weird condition where someone else has the
> page lock, we can avoid calling
> __SetPageMovable().
I think this will suffice:
--- a/mm/z3fold.c~mm-z3foldc-lock-z3fold-page-before-__setpagemovable-fix
+++ a/mm/z3fold.c
@@ -919,6 +919,9 @@ retry:
set_bit(PAGE_HEADLESS, &page->private);
goto headless;
}
+ /*
+ * z3fold_alloc() can be called from atomic contexts, hence the trylock
+ */
if (!WARN_ON(!trylock_page(page))) {
__SetPageMovable(page, pool->inode->i_mapping);
unlock_page(page);
However this code would be more effective if z3fold_alloc() were to be
told when it is running in non-atomic context so it can perform a
sleeping lock_page() in that case. That's an improvement to consider
for later, please.