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Tue, 20 Jun 2023 23:42:15 -0700 (PDT) Received: from gmail.com ([2601:600:8500:5f14:d627:c51e:516e:a105]) by smtp.gmail.com with ESMTPSA id u21-20020a62ed15000000b0063f0068cf6csm2246128pfh.198.2023.06.20.23.42.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Jun 2023 23:42:14 -0700 (PDT) Date: Tue, 20 Jun 2023 23:42:12 -0700 From: Andrei Vagin To: Muhammad Usama Anjum Cc: Peter Xu , David Hildenbrand , Andrew Morton , =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= , Danylo Mocherniuk , Paul Gofman , Cyrill Gorcunov , Mike Rapoport , Nadav Amit , Alexander Viro , Shuah Khan , Christian Brauner , Yang Shi , Vlastimil Babka , "Liam R . Howlett" , Yun Zhou , Suren Baghdasaryan , Alex Sierra , Matthew Wilcox , Pasha Tatashin , Axel Rasmussen , "Gustavo A . R . Silva" , Dan Williams , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, Greg KH , kernel@collabora.com Subject: Re: [PATCH v19 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs Message-ID: References: <20230615141144.665148-1-usama.anjum@collabora.com> <20230615141144.665148-3-usama.anjum@collabora.com> <212e331f-35b0-5ae7-6371-26caa577d637@collabora.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <212e331f-35b0-5ae7-6371-26caa577d637@collabora.com> X-Rspamd-Queue-Id: C8A8C40008 X-Rspam-User: X-Rspamd-Server: rspam04 X-Stat-Signature: xy4qqbyqeftjnwpacnhw68594bwh4ugj X-HE-Tag: 1687329736-531128 X-HE-Meta: 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 Fo36Pv4p 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Jun 19, 2023 at 11:06:36AM +0500, Muhammad Usama Anjum wrote: > On 6/17/23 11:39 AM, Andrei Vagin wrote: > > On Thu, Jun 15, 2023 at 07:11:41PM +0500, Muhammad Usama Anjum wrote: > >> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start, > >> + unsigned long end, struct mm_walk *walk) > >> +{ > >> + bool is_written, flush = false, is_interesting = true; > >> + struct pagemap_scan_private *p = walk->private; > >> + struct vm_area_struct *vma = walk->vma; > >> + unsigned long bitmap, addr = end; > >> + pte_t *pte, *orig_pte, ptent; > >> + spinlock_t *ptl; > >> + int ret = 0; > >> + > >> + arch_enter_lazy_mmu_mode(); > >> + > >> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE > >> + ptl = pmd_trans_huge_lock(pmd, vma); > >> + if (ptl) { > >> + unsigned long n_pages = (end - start)/PAGE_SIZE; > >> + > >> + if (p->max_pages && n_pages > p->max_pages - p->found_pages) > >> + n_pages = p->max_pages - p->found_pages; > >> + > >> + is_written = !is_pmd_uffd_wp(*pmd); > >> + > >> + /* > >> + * Break huge page into small pages if the WP operation need to > >> + * be performed is on a portion of the huge page. > >> + */ > >> + if (is_written && IS_PM_SCAN_WP(p->flags) && > >> + n_pages < HPAGE_SIZE/PAGE_SIZE) { > >> + spin_unlock(ptl); > >> + > >> + split_huge_pmd(vma, pmd, start); > >> + goto process_smaller_pages; > >> + } > >> + > >> + bitmap = PM_SCAN_FLAGS(is_written, (bool)vma->vm_file, > >> + pmd_present(*pmd), is_swap_pmd(*pmd)); > >> + > >> + if (IS_PM_SCAN_GET(p->flags)) { > >> + is_interesting = pagemap_scan_is_interesting_page(bitmap, p); > >> + if (is_interesting) > >> + ret = pagemap_scan_output(bitmap, p, start, n_pages); > >> + } > >> + > >> + if (IS_PM_SCAN_WP(p->flags) && is_written && is_interesting && > >> + ret >= 0) { > >> + make_uffd_wp_pmd(vma, start, pmd); > >> + flush_tlb_range(vma, start, end); > >> + } > >> + > >> + spin_unlock(ptl); > >> + > >> + arch_leave_lazy_mmu_mode(); > >> + return ret; > >> + } > >> + > >> +process_smaller_pages: > >> +#endif > >> + > >> + orig_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, start, &ptl); > >> + if (!pte) { > > > > Do we need to unlock ptl here? > > > > spin_unlock(ptl); > No, please look at these recently merged patches: > https://lore.kernel.org/all/c1c9a74a-bc5b-15ea-e5d2-8ec34bc921d@google.com > > > > >> + walk->action = ACTION_AGAIN; > >> + return 0; > >> + } > >> + > >> + for (addr = start; addr < end && !ret; pte++, addr += PAGE_SIZE) { > >> + ptent = ptep_get(pte); > >> + is_written = !is_pte_uffd_wp(ptent); > >> + > >> + bitmap = PM_SCAN_FLAGS(is_written, (bool)vma->vm_file, > >> + pte_present(ptent), is_swap_pte(ptent)); > > > > The vma->vm_file check isn't correct in this case. You can look when > > pte_to_pagemap_entry sets PM_FILE. This flag is used to detect what > > pages have a file backing store and what pages are anonymous. > I'll update. > > > > > I was trying to integrate this new interace into CRIU and I found > > one more thing that is required. We need to detect zero pages. > Should we name it ZERO_PFN_PRESENT_PAGE to be exact or what? IMHO, ZERO_PFN_PRESENT_PAGE looks a bit monstrous. It looks like zero page is a proper noun in the kernel, so PAGE_IS_ZERO might be a good choice here, but it is up to you. > > > > > It should look something like this: > > > > #define PM_SCAN_FLAGS(wt, file, present, swap, zero) \ > > ((wt) | ((file) << 1) | ((present) << 2) | ((swap) << 3) | ((zero) << 4)) > > > > > > bitmap = PM_SCAN_FLAGS(is_written, page && !PageAnon(page), > > pte_present(ptent), is_swap_pte(ptent), > > pte_present(ptent) && is_zero_pfn(pte_pfn(ptent))); > Okay. Can you please confirm my assumptions: > - A THP cannot be file backed. (PM_FILE isn't being set for THP case) ``` Currently THP only works for anonymous memory mappings and tmpfs/shmem. But in the future it can expand to other filesystems. ``` https://www.kernel.org/doc/html/next/admin-guide/mm/transhuge.html so THP can be "file backed". > - A hole is also not file backed. > > A hole isn't present in memory. So its pfn would be zero. But as it isn't > present, it shouldn't report zero page. Right? For hole:: > > PM_SCAN_FLAGS(false, false, false, false, false) This looks correct to me.