| Message ID | 20200819042722.23414.2654.stgit@localhost.localdomain (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Minor cleanups and performance optimizations for LRU rework | expand |
在 2020/8/19 下午12:27, Alexander Duyck 写道: > From: Alexander Duyck <alexander.h.duyck@linux.intel.com> > > In isolate_lru_pages we have an exception path where if we call > get_page_unless_zero and that succeeds, but TestClearPageLRU fails we call > put_page. Normally this would be problematic but due to the way that the > calls are ordered and the fact that we are holding the LRU lock we know > that the caller must be holding another reference for the page. Since we > can assume that we can replace the put_page with a call to > put_page_testzero contained within a WARN_ON. By doing this we should see > if we ever leak a page as a result of the reference count somehow hitting > zero when it shouldn't, and can avoid the overhead and confusion of using > the full put_page call. > > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com> > --- > mm/vmscan.c | 9 ++++++--- > 1 file changed, 6 insertions(+), 3 deletions(-) > > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 5bc0c2322043..3ebe3f9b653b 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -1688,10 +1688,13 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan, > > if (!TestClearPageLRU(page)) { > /* > - * This page may in other isolation path, > - * but we still hold lru_lock. > + * This page is being isolated in another > + * thread, but we still hold lru_lock. The > + * other thread must be holding a reference > + * to the page so this should never hit a > + * reference count of 0. > */ > - put_page(page); > + WARN_ON(put_page_testzero(page)); seems WARN_ON is always enabled. Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com> > goto busy; > } > >
On Wed, Aug 19, 2020 at 12:52 AM Alex Shi <alex.shi@linux.alibaba.com> wrote: > > > > 在 2020/8/19 下午12:27, Alexander Duyck 写道: > > From: Alexander Duyck <alexander.h.duyck@linux.intel.com> > > > > In isolate_lru_pages we have an exception path where if we call > > get_page_unless_zero and that succeeds, but TestClearPageLRU fails we call > > put_page. Normally this would be problematic but due to the way that the > > calls are ordered and the fact that we are holding the LRU lock we know > > that the caller must be holding another reference for the page. Since we > > can assume that we can replace the put_page with a call to > > put_page_testzero contained within a WARN_ON. By doing this we should see > > if we ever leak a page as a result of the reference count somehow hitting > > zero when it shouldn't, and can avoid the overhead and confusion of using > > the full put_page call. > > > > Signed-off-by: Alexander Duyck <alexander.h.duyck@linux.intel.com> > > --- > > mm/vmscan.c | 9 ++++++--- > > 1 file changed, 6 insertions(+), 3 deletions(-) > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > index 5bc0c2322043..3ebe3f9b653b 100644 > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -1688,10 +1688,13 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan, > > > > if (!TestClearPageLRU(page)) { > > /* > > - * This page may in other isolation path, > > - * but we still hold lru_lock. > > + * This page is being isolated in another > > + * thread, but we still hold lru_lock. The > > + * other thread must be holding a reference > > + * to the page so this should never hit a > > + * reference count of 0. > > */ > > - put_page(page); > > + WARN_ON(put_page_testzero(page)); > > seems WARN_ON is always enabled. > > Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com> Yeah, it is always enabled however it should never be triggered. I had considered just putting a page_ref_dec here since in theory this path should never be triggered but I thought as a debug catch I add the WARN_ON and put_page_testzero. If we ever do encounter this being triggered then it will leak a page of memory which isn't the end of the world but I thought would warrant a WARN_ON.
diff --git a/mm/vmscan.c b/mm/vmscan.c index 5bc0c2322043..3ebe3f9b653b 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -1688,10 +1688,13 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan, if (!TestClearPageLRU(page)) { /* - * This page may in other isolation path, - * but we still hold lru_lock. + * This page is being isolated in another + * thread, but we still hold lru_lock. The + * other thread must be holding a reference + * to the page so this should never hit a + * reference count of 0. */ - put_page(page); + WARN_ON(put_page_testzero(page)); goto busy; }