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Message-ID: <aeaaa33e-4d23-fd3a-1357-4751007aa3bd@collabora.com>
Date: Thu, 1 Jun 2023 13:16:14 +0500
From: Muhammad Usama Anjum <usama.anjum@...labora.com>
To: Peter Xu <peterx@...hat.com>
Cc: Muhammad Usama Anjum <usama.anjum@...labora.com>,
David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Michał Mirosław
<emmir@...gle.com>, Andrei Vagin <avagin@...il.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 v16 2/5] fs/proc/task_mmu: Implement IOCTL to get and
optionally clear info about PTEs
On 6/1/23 2:46 AM, Peter Xu wrote:
> Muhammad,
>
> Sorry, I probably can only review the non-interface part, and leave the
> interface/buffer handling, etc. review for others and real potential users
> of it..
Thank you so much for the review. I think mostly we should be okay with
interface as everybody has been making suggestions over the past revisions.
>
> On Thu, May 25, 2023 at 01:55:14PM +0500, Muhammad Usama Anjum wrote:
>> +static inline void make_uffd_wp_huge_pte(struct vm_area_struct *vma,
>> + unsigned long addr, pte_t *ptep,
>> + pte_t ptent)
>> +{
>> + pte_t old_pte;
>> +
>> + if (!huge_pte_none(ptent)) {
>> + old_pte = huge_ptep_modify_prot_start(vma, addr, ptep);
>> + ptent = huge_pte_mkuffd_wp(old_pte);
>> + ptep_modify_prot_commit(vma, addr, ptep, old_pte, ptent);
>
> huge_ptep_modify_prot_start()?
Sorry, I didn't realized that huge_ptep_modify_prot_start() is different
from its pte version.
>
> The other thing is what if it's a pte marker already? What if a hugetlb
> migration entry? Please check hugetlb_change_protection().
I've updated it in more better way. Please let me know what do you think
about the following:
static inline void make_uffd_wp_huge_pte(struct vm_area_struct *vma,
unsigned long addr, pte_t *ptep,
pte_t ptent)
{
if (is_hugetlb_entry_hwpoisoned(ptent) || is_pte_marker(ptent))
return;
if (is_hugetlb_entry_migration(ptent))
set_huge_pte_at(vma->vm_mm, addr, ptep,
pte_swp_mkuffd_wp(ptent));
else if (!huge_pte_none(ptent))
ptep_modify_prot_commit(vma, addr, ptep, ptent,
huge_pte_mkuffd_wp(ptent));
else
set_huge_pte_at(vma->vm_mm, addr, ptep,
make_pte_marker(PTE_MARKER_UFFD_WP));
}
As we always set UNPOPULATED, so markers are always set on none ptes
initially. Is it possible that a none pte becomes present, then swapped and
finally none again? So I'll do the following addition for make_uffd_wp_pte():
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1800,6 +1800,9 @@ static inline void make_uffd_wp_pte(struct
vm_area_struct *vma,
} else if (is_swap_pte(ptent)) {
ptent = pte_swp_mkuffd_wp(ptent);
set_pte_at(vma->vm_mm, addr, pte, ptent);
+ } else {
+ set_pte_at(vma->vm_mm, addr, pte,
+ make_pte_marker(PTE_MARKER_UFFD_WP));
}
}
>
>> + } else {
>> + set_huge_pte_at(vma->vm_mm, addr, ptep,
>> + make_pte_marker(PTE_MARKER_UFFD_WP));
>> + }
>> +}
>> +#endif
>
> [...]
>
>> +static int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long start,
>> + unsigned long end, struct mm_walk *walk)
>> +{
>> + struct pagemap_scan_private *p = walk->private;
>> + struct vm_area_struct *vma = walk->vma;
>> + unsigned long addr = end;
>> + pte_t *pte, *orig_pte;
>> + spinlock_t *ptl;
>> + bool is_written;
>> + 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;
>> + }
>> +
>> + if (IS_PM_SCAN_GET(p->flags))
>> + ret = pagemap_scan_output(is_written, vma->vm_file,
>> + pmd_present(*pmd),
>> + is_swap_pmd(*pmd),
>> + p, start, n_pages);
>> +
>> + if (ret >= 0 && is_written && IS_PM_SCAN_WP(p->flags))
>> + make_uffd_wp_pmd(vma, addr, pmd);
>> +
>> + if (IS_PM_SCAN_WP(p->flags))
>> + flush_tlb_range(vma, start, end);
>> +
>> + spin_unlock(ptl);
>> +
>> + arch_leave_lazy_mmu_mode();
>> + return ret;
>> + }
>> +
>> +process_smaller_pages:
>> + if (pmd_trans_unstable(pmd)) {
>> + arch_leave_lazy_mmu_mode();
>> + return 0;
>
> I'm not sure whether this is right.. Shouldn't you return with -EAGAIN and
> let the user retry? Returning 0 means you'll move on with the next pmd
> afaict and ignoring this one.
This has come up before. We are just replicating pagemap_pmd_range() here
as we are doing almost the same thing through IOCTL. It doesn't return any
error in this case and just skips it. So we are doing the same.
>
>> + }
>> +#endif
>> +
>> + orig_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, start, &ptl);
>
> Just a heads-up that this may start to fail at some point if Hugh's work
> will land earlier:
>
> https://lore.kernel.org/linux-mm/68a97fbe-5c1e-7ac6-72c-7b9c6290b370@google.com/
Thank you so much for the heads up.
>
>> + for (addr = start; addr < end && !ret; pte++, addr += PAGE_SIZE) {
>> + is_written = !is_pte_uffd_wp(*pte);
>> +
>> + if (IS_PM_SCAN_GET(p->flags))
>> + ret = pagemap_scan_output(is_written, vma->vm_file,
>> + pte_present(*pte),
>> + is_swap_pte(*pte),
>> + p, addr, 1);
>> +
>> + if (ret >= 0 && is_written && IS_PM_SCAN_WP(p->flags))
>> + make_uffd_wp_pte(vma, addr, pte);
>> + }
>> +
>> + if (IS_PM_SCAN_WP(p->flags))
>> + flush_tlb_range(vma, start, addr);
>> +
>> + pte_unmap_unlock(orig_pte, ptl);
>> + arch_leave_lazy_mmu_mode();
>> +
>> + cond_resched();
>> + return ret;
>> +}
>> +
>> +#ifdef CONFIG_HUGETLB_PAGE
>> +static int pagemap_scan_hugetlb_entry(pte_t *ptep, unsigned long hmask,
>> + unsigned long start, unsigned long end,
>> + struct mm_walk *walk)
>> +{
>> + unsigned long n_pages = (end - start)/PAGE_SIZE;
>> + struct pagemap_scan_private *p = walk->private;
>> + struct vm_area_struct *vma = walk->vma;
>> + struct hstate *h = hstate_vma(vma);
>> + spinlock_t *ptl;
>> + bool is_written;
>> + int ret = 0;
>> + pte_t pte;
>> +
>> + arch_enter_lazy_mmu_mode();
>
> This _seems_ to be not needed for hugetlb entries.
I'll remove it.
>
>> +
>> + if (p->max_pages && n_pages > p->max_pages - p->found_pages)
>> + n_pages = p->max_pages - p->found_pages;
>> +
>> + if (IS_PM_SCAN_WP(p->flags)) {
>> + i_mmap_lock_write(vma->vm_file->f_mapping);
>> + ptl = huge_pte_lock(h, vma->vm_mm, ptep);
>> + }
>> +
>> + pte = huge_ptep_get(ptep);
>> + is_written = !is_huge_pte_uffd_wp(pte);
>> +
>> + /*
>> + * Partial hugetlb page clear isn't supported
>> + */
>> + if (is_written && IS_PM_SCAN_WP(p->flags) &&
>> + n_pages < HPAGE_SIZE/PAGE_SIZE) {
>> + ret = -EPERM;
>> + goto unlock_and_return;
>> + }
>> +
>> + if (IS_PM_SCAN_GET(p->flags)) {
>> + ret = pagemap_scan_output(is_written, vma->vm_file,
>> + pte_present(pte), is_swap_pte(pte),
>> + p, start, n_pages);
>> + if (ret < 0)
>> + goto unlock_and_return;
>> + }
>> +
>> + if (is_written && IS_PM_SCAN_WP(p->flags)) {
>> + make_uffd_wp_huge_pte(vma, start, ptep, pte);
>> + flush_hugetlb_tlb_range(vma, start, end);
>> + }
>> +
>> +unlock_and_return:
>> + if (IS_PM_SCAN_WP(p->flags)) {
>> + spin_unlock(ptl);
>> + i_mmap_unlock_write(vma->vm_file->f_mapping);
>> + }
>> +
>> + arch_leave_lazy_mmu_mode();
>
> Same here.
>
>> +
>> + return ret;
>> +}
>
> [...]
>
--
BR,
Muhammad Usama Anjum
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