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Date: Wed, 24 Aug 2022 06:49:52 +0000 From: Christophe Leroy <christophe.leroy@...roup.eu> To: Zhouyi Zhou <zhouzhouyi@...il.com> CC: "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, "nathan@...nel.org" <nathan@...nel.org>, "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>, "trix@...hat.com" <trix@...hat.com>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "llvm@...ts.linux.dev" <llvm@...ts.linux.dev> Subject: Re: [PATCH linux-next] mm: fix used but uninitialized variable Le 24/08/2022 à 08:41, Zhouyi Zhou a écrit : > Thank Christophe for reviewing my patch > > On Wed, Aug 24, 2022 at 2:10 PM Christophe Leroy > <christophe.leroy@...roup.eu> wrote: >> >> >> >> Le 24/08/2022 à 07:56, Zhouyi Zhou a écrit : >>> In function walk_hugetlb_range, the local variable err may >>> be used uninitialzed when: >>> ops->pte_hole in side of "else if (ops->pte_hole)" is false. >>> >>> Signed-off-by: Zhouyi Zhou <zhouzhouyi@...il.com> >>> --- >>> Dear mm Developers: >>> >>> When I build kernel using "make CC=clang-14" >>> the compiler complains following: >>> >>> CC mm/pagewalk.o >>> mm/pagewalk.c:318:12: error: variable 'err' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized] >>> else if (ops->pte_hole) >>> ^~~~~~~~~~~~~ >>> mm/pagewalk.c:321:7: note: uninitialized use occurs here >>> if (err) >>> ^~~ >>> mm/pagewalk.c:318:8: note: remove the 'if' if its condition is always true >>> else if (ops->pte_hole) >>> ^~~~~~~~~~~~~~~~~~ >>> mm/pagewalk.c:311:10: note: initialize the variable 'err' to silence this warning >>> int err; >>> ^ >>> = 0 >>> 1 error generated. >>> make[1]: *** [scripts/Makefile.build:250: mm/pagewalk.o] Error 1 >>> make: *** [Makefile:2006: mm] Error 2 >>> >>> I initialize that variable outside of the for loop because we can assign 0 to err >>> only once in this function. >>> >>> After my fix, I can compile the kernel. >>> >>> Many Thanks >>> Zhouyi >>> -- >>> mm/pagewalk.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/mm/pagewalk.c b/mm/pagewalk.c >>> index 54b2a1beeeb3..b6eb330e8ecd 100644 >>> --- a/mm/pagewalk.c >>> +++ b/mm/pagewalk.c >>> @@ -306,9 +306,9 @@ static int walk_hugetlb_range(unsigned long addr, unsigned long end, >>> unsigned long hmask = huge_page_mask(h); >>> unsigned long sz = huge_page_size(h); >>> const struct mm_walk_ops *ops = walk->ops; >>> + int err = 0; >> >> Why do you move it back outside of the for loop allthough it is >> exclusively used inside the loop ? > I move it outside of the for loop for performance consideration. Because > if we initialize err inside, there will be an assignment statement > every iteration. I think GCC is smart enough to do the assignment only when necessary, maybe have a look at the generated assembly in order to confirm. >> >>> >>> for (; addr < end; addr = next) { >>> - int err; >> >> Another solution would be to add an explicit else, setting err = 0 in >> the if/else if sequence. > Thank Christophe for your valuable advice, I am going to prepare a 2nd version Don't spend too much time on that, there are already other people looking at it, see https://lore.kernel.org/linux-mm/20220823153055.2517764-1-nathan@kernel.org/T/ > > Thanks > Zhouyi >> >>> pte_t *pte = huge_pte_offset(walk->mm, addr & hmask, sz); >>> >>> next = hugetlb_entry_end(h, addr, end);
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