From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id E44251FDD for ; Tue, 1 Jul 2025 08:30:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751358661; cv=none; b=q5mFwr+HpXpQI75VEjTJbTLFjL4SWJxg9wZ09TyMTckXkLqemOmxvqJ4acBI7bIa36tTxIs6Q388w4+LGB6iwRZsNtIk02YkvI7WBoTHLdXfJKwgQXSK36vqcqTlDFtf7NwDWothlbIhcUN3/E916Lc24LQpZs4n6ntActe4NkM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1751358661; c=relaxed/simple; bh=BDUQlWmZIDNUFkjpMGf0JQSl/ZqDNpstCpGILV7Wmec=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=FXsent4+i+8r3zqyly1oMAKdy53xsMk8tl+FJKSwE+9UYTcA+nccVMwEwJ41jmW2P1oKdEBtmKgfA0h7AohoONuRae97+UZESyJVJUHiy7EYr3CUjmQAisjXML+AVbZzfZoQrZgBEyDvSH2LlYfUyD1dYvjYI1IiGlJ5YcdFd7E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 9D7A51595; Tue, 1 Jul 2025 01:30:43 -0700 (PDT) Received: from [10.57.84.129] (unknown [10.57.84.129]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EF9E63F58B; Tue, 1 Jul 2025 01:30:53 -0700 (PDT) Message-ID: <3776ac82-4cd5-41e7-9b96-137a1ae6f473@arm.com> Date: Tue, 1 Jul 2025 09:30:51 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 3/4] mm: Optimize mprotect() by PTE-batching Content-Language: en-GB To: Lorenzo Stoakes Cc: Dev Jain , akpm@linux-foundation.org, david@redhat.com, willy@infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, catalin.marinas@arm.com, will@kernel.org, Liam.Howlett@oracle.com, vbabka@suse.cz, jannh@google.com, anshuman.khandual@arm.com, peterx@redhat.com, joey.gouly@arm.com, ioworker0@gmail.com, baohua@kernel.org, kevin.brodsky@arm.com, quic_zhenhuah@quicinc.com, christophe.leroy@csgroup.eu, yangyicong@hisilicon.com, linux-arm-kernel@lists.infradead.org, hughd@google.com, yang@os.amperecomputing.com, ziy@nvidia.com References: <20250628113435.46678-1-dev.jain@arm.com> <20250628113435.46678-4-dev.jain@arm.com> <0315c016-d707-42b9-8038-7a2d5e4387f6@lucifer.local> <5900e978-6349-4a3d-a384-758889b678a0@lucifer.local> From: Ryan Roberts In-Reply-To: <5900e978-6349-4a3d-a384-758889b678a0@lucifer.local> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 01/07/2025 09:15, Lorenzo Stoakes wrote: > On Tue, Jul 01, 2025 at 09:03:27AM +0100, Ryan Roberts wrote: >>> >>> ////// end of Lorenzo's suggestion ////// >>> >>> You can obviously modify this to change other stuff like whether you feed back >>> the PAE or not in private case for use in your code. >> >> This sugestion for this part of the problem looks much cleaner! > > Thanks :) > >> >> Sorry; this whole struct tristate thing was my idea. I never really liked it but >> I was more focussed on trying to illustrate the big picture flow that I thought >> would work well with a batch and sub-batches (which it seems below that you >> hate... but let's talk about that down there). > > Yeah, this is fiddly stuff so I get it as a sort of psuedocode, but as code > obviously I've made my feelings known haha. > > It seems that we can apply the fundamental underlying idea without needing to do > it this way at any rate so we should be good. > >>>> >>>> static int mprotect_folio_pte_batch(struct folio *folio, unsigned long addr, >>>> - pte_t *ptep, pte_t pte, int max_nr_ptes) >>>> + pte_t *ptep, pte_t pte, int max_nr_ptes, fpb_t switch_off_flags) >>> >>> This last parameter is pretty horrible. It's a negative mask so now you're >>> passing 'ignore soft dirty' to the function meaning 'don't ignore it'. This is >>> just planting land mines. >>> >>> Obviously David's flag changes will also alter all this. >>> >>> Just add a boolean re: soft dirty. >> >> Dev had a boolean for this in the last round. I've seen various functions expand >> over time with increasing numbers of bool flags. So I asked to convert to a >> flags parameter and just pass in the flags we need. Then it's a bit more future >> proof and self documenting. (For the record I dislike the "switch_off_flags" >> approach taken here). > > Yeah, but we can change this when it needs to be changed. When it comes to > internal non-uAPI stuff I don't think we need to be too worried about > future-proofing like this at least just yet. > > Do not fear the future churn... ;) > > I mean I guess David's new flags will make this less egregious anyway. > >>>> oldpte = modify_prot_start_ptes(vma, addr, pte, nr_ptes); >>>> ptent = pte_modify(oldpte, newprot); >>>> >>>> @@ -227,15 +294,39 @@ static long change_pte_range(struct mmu_gather *tlb, >>>> * example, if a PTE is already dirty and no other >>>> * COW or special handling is required. >>>> */ >>>> - if ((cp_flags & MM_CP_TRY_CHANGE_WRITABLE) && >>>> - !pte_write(ptent) && >>>> - can_change_pte_writable(vma, addr, ptent)) >>>> - ptent = pte_mkwrite(ptent, vma); >>>> - >>>> - modify_prot_commit_ptes(vma, addr, pte, oldpte, ptent, nr_ptes); >>>> - if (pte_needs_flush(oldpte, ptent)) >>>> - tlb_flush_pte_range(tlb, addr, PAGE_SIZE); >>>> - pages++; >>>> + set_write = (cp_flags & MM_CP_TRY_CHANGE_WRITABLE) && >>>> + !pte_write(ptent); >>>> + if (set_write) >>>> + set_write = maybe_change_pte_writable(vma, addr, ptent, folio); >>>> + >>>> + while (nr_ptes) { >>>> + if (set_write == TRI_MAYBE) { >>>> + sub_nr_ptes = anon_exclusive_batch(folio, >>>> + pgidx, nr_ptes, &sub_set_write); >>>> + } else { >>>> + sub_nr_ptes = nr_ptes; >>>> + sub_set_write = (set_write == TRI_TRUE); >>>> + } >>>> + >>>> + if (sub_set_write) >>>> + newpte = pte_mkwrite(ptent, vma); >>>> + else >>>> + newpte = ptent; >>>> + >>>> + modify_prot_commit_ptes(vma, addr, pte, oldpte, >>>> + newpte, sub_nr_ptes); >>>> + if (pte_needs_flush(oldpte, newpte)) >>>> + tlb_flush_pte_range(tlb, addr, >>>> + sub_nr_ptes * PAGE_SIZE); >>>> + >>>> + addr += sub_nr_ptes * PAGE_SIZE; >>>> + pte += sub_nr_ptes; >>>> + oldpte = pte_advance_pfn(oldpte, sub_nr_ptes); >>>> + ptent = pte_advance_pfn(ptent, sub_nr_ptes); >>>> + nr_ptes -= sub_nr_ptes; >>>> + pages += sub_nr_ptes; >>>> + pgidx += sub_nr_ptes; >>>> + } >>> >>> I hate hate hate having this loop here, let's abstract this please. >>> >>> I mean I think we can just use mprotect_folio_pte_batch() no? It's not >>> abstracting much here, and we can just do all this handling there. Maybe have to >>> pass in a bunch more params, but it saves us having to do all this. >> >> In an ideal world we would flatten and just have mprotect_folio_pte_batch() >> return the batch size considering all the relevant PTE bits AND the >> AnonExclusive bit on the pages. IIRC one of Dev's earlier versions modified the >> core folio_pte_batch() function to also look at the AnonExclusive bit, but I >> really disliked changing that core function (I think others did too?). > > Yeah let's not change the core function. > > My suggestion is to have mprotect_folio_pte_batch() do this. > >> >> So barring that approach, we are really only left with the batch and sub-batch >> approach - although, yes, it could be abstracted more. We could maintain a >> context struct that persists across all calls to mprotect_folio_pte_batch() and >> it can use that to keep it's state to remember if we are in the middle of a >> sub-batch and decide either to call folio_pte_batch() to get a new batch, or >> call anon_exclusive_batch() to get the next sub-batch within the current batch. >> But that started to feel overly abstracted to me. > > Having this nested batch/sub-batch loop really feels worse. You just get lost in > the complexity here very easily. > > But i"m also not sure we need to maintain _that_ much state? > > We're already looping over all of the PTEs here, so abstracting _the entire > loop_ and all the sub-batch stuff to another function, that is > mprotect_folio_pte_batch() I think sensibly, so it handles this for you makes a > ton of sense. So effectively turn mprotect_folio_pte_batch() into an iterator; have a struct and a funtion to init the struct for the the number of ptes we want to iterate over, then a per iteration function that progressively returns batches? Then we just have a simple loop here that gets the next batch and processes it? > >> >> This loop approach, as written, felt more straightforward for the reader to >> understand (i.e. the least-worst option). Is the context approach what you are >> suggesting or do you have something else in mind? >> > > See above. > >>> >>> Alternatively, we could add a new wrapper function, but yeah definitely not >>> this. >>> >>> Also the C programming language asks... etc etc. ;) >>> >>> Overall since you always end up processing folio_nr_pages(folio) you can just >>> have the batch function or a wrapper return this and do updates as necessary >>> here on that basis, and leave the 'sub' batching to that function. >> >> Sorry I don't understand this statement - could you clarify? Especially the bit >> about "always ... processing folio_nr_pages(folio)"; I don't think we do. In >> various corner cases the size of the folio has no relationship to the way the >> PTEs are mapped. > > Right yeah I put this badly. Obviously you can have all sorts of fun with large > folios partially mapped and page-table split and etc. etc. > > I should have said 'always process nr_ptes'. > > The idea is to abstract this sub-batch stuff to another function, fundamentally.