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Message-ID: <ZKLkIpL7Ksr38VRb@gmail.com>
Date: Mon, 3 Jul 2023 08:07:14 -0700
From: Andrei Vagin <avagin@...il.com>
To: Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc: Peter Xu <peterx@...hat.com>, David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Michał Mirosław <emmir@...gle.com>,
Danylo Mocherniuk <mdanylo@...gle.com>,
Paul Gofman <pgofman@...eweavers.com>,
Cyrill Gorcunov <gorcunov@...il.com>,
Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Shuah Khan <shuah@...nel.org>,
Christian Brauner <brauner@...nel.org>,
Yang Shi <shy828301@...il.com>,
Vlastimil Babka <vbabka@...e.cz>,
"Liam R . Howlett" <Liam.Howlett@...cle.com>,
Yun Zhou <yun.zhou@...driver.com>,
Suren Baghdasaryan <surenb@...gle.com>,
Alex Sierra <alex.sierra@....com>,
Matthew Wilcox <willy@...radead.org>,
Pasha Tatashin <pasha.tatashin@...een.com>,
Axel Rasmussen <axelrasmussen@...gle.com>,
"Gustavo A . R . Silva" <gustavoars@...nel.org>,
Dan Williams <dan.j.williams@...el.com>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com
Subject: Re: [PATCH v22 2/5] fs/proc/task_mmu: Implement IOCTL to get and
optionally clear info about PTEs
On Mon, Jul 03, 2023 at 11:47:37AM +0500, Muhammad Usama Anjum wrote:
> On 6/30/23 8:01 PM, Andrei Vagin wrote:
> > On Wed, Jun 28, 2023 at 02:54:23PM +0500, Muhammad Usama Anjum wrote:
> >> This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
> >> the info about page table entries. The following operations are supported
> >> in this ioctl:
> >> - Get the information if the pages have been written-to (PAGE_IS_WRITTEN),
> >> file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT), swapped
> >> (PAGE_IS_SWAPPED) or page has pfn zero (PAGE_IS_PFNZERO).
> >> - Find pages which have been written-to and/or write protect the pages
> >> (atomic PM_SCAN_OP_GET + PM_SCAN_OP_WP)
> >>
> >> This IOCTL can be extended to get information about more PTE bits. The
> >> entire address range passed by user [start, end) is scanned until either
> >> the user provided buffer is full or max_pages have been found.
> >>
> >> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>
> >
> > <snip>
> >
> >> +
> >> +static long do_pagemap_scan(struct mm_struct *mm, unsigned long __arg)
> >> +{
> >> + struct pm_scan_arg __user *uarg = (struct pm_scan_arg __user *)__arg;
> >> + unsigned long long start, end, walk_start, walk_end;
> >> + unsigned long empty_slots, vec_index = 0;
> >> + struct mmu_notifier_range range;
> >> + struct page_region __user *vec;
> >> + struct pagemap_scan_private p;
> >> + struct pm_scan_arg arg;
> >> + int ret = 0;
> >> +
> >> + if (copy_from_user(&arg, uarg, sizeof(arg)))
> >> + return -EFAULT;
> >> +
> >> + start = untagged_addr((unsigned long)arg.start);
> >> + end = untagged_addr((unsigned long)arg.end);
> >> + vec = (struct page_region __user *)untagged_addr((unsigned long)arg.vec);
> >> +
> >> + ret = pagemap_scan_args_valid(&arg, start, end, vec);
> >> + if (ret)
> >> + return ret;
> >> +
> >> + p.max_pages = (arg.max_pages) ? arg.max_pages : ULONG_MAX;
> >> + p.found_pages = 0;
> >> + p.required_mask = arg.required_mask;
> >> + p.anyof_mask = arg.anyof_mask;
> >> + p.excluded_mask = arg.excluded_mask;
> >> + p.return_mask = arg.return_mask;
> >> + p.flags = arg.flags;
> >> + p.flags |= ((p.required_mask | p.anyof_mask | p.excluded_mask) &
> >> + PAGE_IS_WRITTEN) ? PM_SCAN_REQUIRE_UFFD : 0;
> >> + p.cur_buf.start = p.cur_buf.len = p.cur_buf.flags = 0;
> >> + p.vec_buf = NULL;
> >> + p.vec_buf_len = PAGEMAP_WALK_SIZE >> PAGE_SHIFT;
> >> +
> >> + /*
> >> + * Allocate smaller buffer to get output from inside the page walk
> >> + * functions and walk page range in PAGEMAP_WALK_SIZE size chunks. As
> >> + * we want to return output to user in compact form where no two
> >> + * consecutive regions should be continuous and have the same flags.
> >> + * So store the latest element in p.cur_buf between different walks and
> >> + * store the p.cur_buf at the end of the walk to the user buffer.
> >> + */
> >> + if (IS_PM_SCAN_GET(p.flags)) {
> >> + p.vec_buf = kmalloc_array(p.vec_buf_len, sizeof(*p.vec_buf),
> >> + GFP_KERNEL);
> >> + if (!p.vec_buf)
> >> + return -ENOMEM;
> >> + }
> >> +
> >> + if (IS_PM_SCAN_WP(p.flags)) {
> >> + mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_VMA, 0,
> >> + mm, start, end);
> >> + mmu_notifier_invalidate_range_start(&range);
> >> + }
> >> +
> >> + walk_start = walk_end = start;
> >> + while (walk_end < end && !ret) {
> >> + if (IS_PM_SCAN_GET(p.flags)) {
> >> + p.vec_buf_index = 0;
> >> +
> >> + /*
> >> + * All data is copied to cur_buf first. When more data
> >> + * is found, we push cur_buf to vec_buf and copy new
> >> + * data to cur_buf. Subtract 1 from length as the
> >> + * index of cur_buf isn't counted in length.
> >> + */
> >> + empty_slots = arg.vec_len - vec_index;
> >> + p.vec_buf_len = min(p.vec_buf_len, empty_slots - 1);
> >> + }
> >> +
> >> + ret = mmap_read_lock_killable(mm);
> >> + if (ret)
> >> + goto return_status;
> >> +
> >> + walk_end = min((walk_start + PAGEMAP_WALK_SIZE) & PAGEMAP_WALK_MASK, end);
> >> +
> >> + ret = walk_page_range(mm, walk_start, walk_end,
> >> + &pagemap_scan_ops, &p);
> >> + mmap_read_unlock(mm);
> >> +
> >> + if (ret && ret != PM_SCAN_FOUND_MAX_PAGES &&
> >> + ret != PM_SCAN_END_WALK)
> >> + goto return_status;
> >> +
> >> + walk_start = walk_end;
> >> + if (IS_PM_SCAN_GET(p.flags) && p.vec_buf_index) {
> >> + if (copy_to_user(&vec[vec_index], p.vec_buf,
> >> + p.vec_buf_index * sizeof(*p.vec_buf))) {
> >> + /*
> >> + * Return error even though the OP succeeded
> >> + */
> >> + ret = -EFAULT;
> >> + goto return_status;
> >> + }
> >> + vec_index += p.vec_buf_index;
> >> + }
> >> + }
> >> +
> >> + if (p.cur_buf.len) {
> >> + if (copy_to_user(&vec[vec_index], &p.cur_buf, sizeof(p.cur_buf))) {
> >> + ret = -EFAULT;
> >> + goto return_status;
> >> + }
> >> + vec_index++;
> >> + }
> >> +
> >> + ret = vec_index;
> >> +
> >> +return_status:
> >> + arg.start = (unsigned long)walk_end;
> >
> > This doesn't look right. pagemap_scan_pmd_entry can stop early. For
> > example, it can happen when it hits the max_pages limit. Do I miss
> > something?
> The walk_page_range() calls pagemap_scan_pmd_entry(). So whatever status is
> returned from pagemap_scan_pmd_entry(), walk_page_range() returns to this
> function where we are handling the status code. After while loop starts,
> there is only 1 return path. Hence there isn't any path missing where we'll
> miss setting arg.start.
I mean walk_end isn't actually the end address. The end adress should be
the next page after the last revised page. Here we don't know whether
all pages in [walk_start, walk_end) has been revised.
>
> >
> >> + if (copy_to_user(&uarg->start, &arg.start, sizeof(arg.start)))
> >> + ret = -EFAULT;
> >> +
> >> + if (IS_PM_SCAN_WP(p.flags))
> >> + mmu_notifier_invalidate_range_end(&range);
> >> +
> >> + kfree(p.vec_buf);
> >> + return ret;
> >> +}
> >> +
>
> --
> BR,
> Muhammad Usama Anjum
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