On Thu 04-04-24 16:16:15, Suren Baghdasaryan wrote: > On Thu, Apr 4, 2024 at 4:01 PM Kent Overstreet > <kent.overstreet@xxxxxxxxx> wrote: > > > > On Thu, Apr 04, 2024 at 03:41:50PM -0700, Andrew Morton wrote: > > > On Thu, 4 Apr 2024 18:38:39 -0400 Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote: > > > > > > > On Thu, Apr 04, 2024 at 11:33:22PM +0100, Matthew Wilcox wrote: > > > > > On Thu, Apr 04, 2024 at 03:17:43PM -0700, Suren Baghdasaryan wrote: > > > > > > Ironically, checkpatch generates warnings for these type casts: > > > > > > > > > > > > WARNING: unnecessary cast may hide bugs, see > > > > > > http://c-faq.com/malloc/mallocnocast.html > > > > > > #425: FILE: include/linux/dma-fence-chain.h:90: > > > > > > + ((struct dma_fence_chain *)kmalloc(sizeof(struct dma_fence_chain), > > > > > > GFP_KERNEL)) > > > > > > > > > > > > I guess I can safely ignore them in this case (since we cast to the > > > > > > expected type)? > > > > > > > > > > I find ignoring checkpatch to be a solid move 99% of the time. > > > > > > > > > > I really don't like the codetags. This is so much churn, and it could > > > > > all be avoided by just passing in _RET_IP_ or _THIS_IP_ depending on > > > > > whether we wanted to profile this function or its caller. vmalloc > > > > > has done it this way since 2008 (OK, using __builtin_return_address()) > > > > > and lockdep has used _THIS_IP_ / _RET_IP_ since 2006. > > > > > > > > Except you can't. We've been over this; using that approach for tracing > > > > is one thing, using it for actual accounting isn't workable. > > > > > > I missed that. There have been many emails. Please remind us of the > > > reasoning here. > > > > I think it's on the other people claiming 'oh this would be so easy if > > you just do it this other way' to put up some code - or at least more > > than hot takes. > > > > But, since you asked - one of the main goals of this patchset was to be > > fast enough to run in production, and if you do it by return address > > then you've added at minimum a hash table lookup to every allocate and > > free; if you do that, running it in production is completely out of the > > question. > > > > Besides that - the issues with annotating and tracking the correct > > callsite really don't go away, they just shift around a bit. It's true > > that the return address approach would be easier initially, but that's > > not all we're concerned with; we're concerned with making sure > > allocations get accounted to the _correct_ callsite so that we're giving > > numbers that you can trust, and by making things less explicit you make > > that harder. > > > > Additionally: the alloc_hooks() macro is for more than this. It's also > > for more usable fault injection - remember every thread we have where > > people are begging for every allocation to be __GFP_NOFAIL - "oh, error > > paths are hard to test, let's just get rid of them" - never mind that > > actually do have to have error paths - but _per callsite_ selectable > > fault injection will actually make it practical to test memory error > > paths. > > > > And Kees working on stuff that'll make use of the alloc_hooks() macro > > for segregating kmem_caches. > > Yeah, that pretty much summarizes it. Note that we don't have to make > the conversions in this patch and accounting will still work but then > all allocations from different callers will be accounted to the helper > function and that's less useful than accounting at the call site. > It's a sizable churn but the conversions are straight-forward and we > do get accurate, performant and easy to use memory accounting. OK, fair enough. I guess I can live with the allocation macros in jbd2 if type safety is preserved. But please provide a short summary of why we need these macros (e.g. instead of RET_IP approach) in the changelog (or at least a link to some email explaining this if the explanation would get too long). Because I was wondering about the same as Andrew (and yes, this is because I wasn't really following the huge discussion last time). Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
