| Message ID | 7a4f5e5e-de33-dace-c526-4a3d3cf5f6e0@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | [1/3,fix] mm,thp,rmap: subpages_mapcount of PTE-mapped subpages: fix | expand |
On Fri, Nov 18, 2022 at 05:35:05PM -0800, Hugh Dickins wrote: > Yu Zhao reports compiler warning in page_add_anon_rmap(): > > mm/rmap.c:1236:13: warning: variable 'first' is used uninitialized > whenever 'if' condition is false [-Wsometimes-uninitialized] > } else if (PageTransHuge(page)) { > ^~~~~~~~~~~~~~~~~~~ > mm/rmap.c:1248:18: note: uninitialized use occurs here > VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page); > ^~~~~ > > We do need to fix that, even though it's only uninitialized in an > impossible condition: I've chosen to initialize "first" true, to > minimize the BUGs it might then hit; but you could just as well > choose to initialize it false, to maximize the BUGs it might hit. > > Reported-by: Yu Zhao <yuzhao@google.com> > Signed-off-by: Hugh Dickins <hughd@google.com> > --- > mm/rmap.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index 66be8cae640f..25b720d5ba17 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1281,7 +1281,7 @@ void page_add_anon_rmap(struct page *page, > struct compound_mapcounts mapcounts; > int nr = 0, nr_pmdmapped = 0; > bool compound = flags & RMAP_COMPOUND; > - bool first; > + bool first = true; > > if (unlikely(PageKsm(page))) > lock_page_memcg(page); Other option is to drop PageTransHuge() check that you already claim to be redundant. Or have else BUG() to catch cases where the helper called with compound=true on non-THP page.
On Mon, 21 Nov 2022, Kirill A. Shutemov wrote: > On Fri, Nov 18, 2022 at 05:35:05PM -0800, Hugh Dickins wrote: ... > > --- a/mm/rmap.c > > +++ b/mm/rmap.c > > @@ -1281,7 +1281,7 @@ void page_add_anon_rmap(struct page *page, > > struct compound_mapcounts mapcounts; > > int nr = 0, nr_pmdmapped = 0; > > bool compound = flags & RMAP_COMPOUND; > > - bool first; > > + bool first = true; > > > > if (unlikely(PageKsm(page))) > > lock_page_memcg(page); > > Other option is to drop PageTransHuge() check that you already claim to be > redundant. > > Or have else BUG() to catch cases where the helper called with > compound=true on non-THP page. I'm sticking with the "first = true". I did receive a report of bloating some tiny config a little, on the very first series, so I've been on guard since then about adding THP code where the optimizer cannot see to remove it: so do want to keep the PageTransHuge check in there. Could be done in other ways, yes, but I'd ended up feeling this was the best compromise. Hugh
diff --git a/mm/rmap.c b/mm/rmap.c index 66be8cae640f..25b720d5ba17 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -1281,7 +1281,7 @@ void page_add_anon_rmap(struct page *page, struct compound_mapcounts mapcounts; int nr = 0, nr_pmdmapped = 0; bool compound = flags & RMAP_COMPOUND; - bool first; + bool first = true; if (unlikely(PageKsm(page))) lock_page_memcg(page);
Yu Zhao reports compiler warning in page_add_anon_rmap(): mm/rmap.c:1236:13: warning: variable 'first' is used uninitialized whenever 'if' condition is false [-Wsometimes-uninitialized] } else if (PageTransHuge(page)) { ^~~~~~~~~~~~~~~~~~~ mm/rmap.c:1248:18: note: uninitialized use occurs here VM_BUG_ON_PAGE(!first && (flags & RMAP_EXCLUSIVE), page); ^~~~~ We do need to fix that, even though it's only uninitialized in an impossible condition: I've chosen to initialize "first" true, to minimize the BUGs it might then hit; but you could just as well choose to initialize it false, to maximize the BUGs it might hit. Reported-by: Yu Zhao <yuzhao@google.com> Signed-off-by: Hugh Dickins <hughd@google.com> --- mm/rmap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)