Hi Catalin, On Fri, Jun 25, 2021 at 04:01:33PM +0100, Catalin Marinas wrote: > On Thu, Jun 24, 2021 at 10:36:50AM -0700, Rustam Kovhaev wrote: > > On Wed, Jun 16, 2021 at 11:25:22AM -0700, Rustam Kovhaev wrote: > > > On Tue, Jun 15, 2021 at 07:15:24AM +0200, Dmitry Vyukov wrote: > > > > On Mon, Jun 14, 2021 at 10:31 PM Rustam Kovhaev <rkovhaev@xxxxxxxxx> wrote: > > > > > > > > > > hello Catalin, Andrew! > > > > > > > > > > while troubleshooting a false positive syzbot kmemleak report i have > > > > > noticed an interesting behavior in kmemleak and i wonder whether it is > > > > > behavior by design and should be documented, or maybe something to > > > > > improve. > > > > > apologies if some of the questions do not make sense, i am still going > > > > > through kmemleak code.. > > > > > > > > > > a) kmemleak scans struct page (kmemleak.c:1462), but it does not scan > > > > > the actual contents (page_address(page)) of the page. > > > > > if we allocate an object with kmalloc(), then allocate page with > > > > > alloc_page(), and if we put kmalloc pointer somewhere inside that page, > > > > > kmemleak will report kmalloc pointer as a false positive. > > > > > should we improve kmemleak and make it scan page contents? > > > > > or will this bring too many false negatives? > > > > > > > > Hi Rustam, > > > > > > > > Nice debugging! > > > > I assume lots of pages are allocated for slab and we don't want to > > > > scan the whole page if only a few slab objects are alive on the page. > > > > However alloc_pages() can be called by end kernel code as well. > > > > I grepped for any kmemleak annotations around existing calls to > > > > alloc_pages, but did not find any... > > > > Does it require an explicit kmemleak_alloc() after allocating the page > > > > and kmemleak_free () before freeing the page? > > > > > > hi Dmitry, thank you! > > > yes, as Catalin has pointed out, there are a few places where we call > > > kmemleak_alloc()/kmemleak_free() explicitly in order for the pages to be > > > scanned, like in blk_mq_alloc_rqs() > > > > > > > If there are more than one use case for this, I guess we could add > > > > some GFP flag for this maybe. > > > > > > and this way kernel users won't have to use kmemleak fuctions mentioned > > > above including some or most kmemleak_not_leak() calls and basically > > > kmemleak will be kind of "transparent" to them? and they will only need > > > to use the GFP flag to instruct kmemleak to scan the page contents? > > > it sounds like a good idea to me.. > > > > > > > i've been thinking about this and it seems like in the scenario where we > > want kmemleak to scan only some part of the page, we will have to either > > do separate alloc_page() calls with different flags or use > > kmemleak_scan_area() to limit the memory scan area. maybe this approach > > won't simplify things and will produce more code instead of reducing it > > Since page allocation is not tracked by kmemleak, you can always do an > explicit kmemleak_alloc() call with a smaller size than a full page. > right, but if i understood Dmitry's idea correctly, he was thinking about using a new GFP flag, like GFP_KMEMLEAK, and burying kmemleak_alloc() in page allocator
