Re: [PATCH v6 0/3] Implement IOCTL to get and/or the clear info about PTEs

David Hildenbrand
2022-11-21T15:57:08Z

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On 21.11.22 16:00, Muhammad Usama Anjum wrote:
> Hello,
>
> Thank you for replying.
>
> On 11/14/22 8:46 PM, David Hildenbrand wrote:
>>> The soft-dirtiness is stored in the PTE. VMA is marked dirty to store the
>>> dirtiness for reused regions. Clearing the soft-dirty status of whole
>>> process is straight forward. When we want to clear/monitor the
>>> soft-dirtiness of a part of the virtual memory, there is a lot of internal
>>> noise. We don't want the non-dirty pages to become dirty because of how the
>>> soft-dirty feature has been working. Soft-dirty feature wasn't being used
>>> the way we want to use now. While monitoring a part of memory, it is not
>>> acceptable to get non-dirty pages as dirty. Non-dirty pages become dirty
>>> when the two VMAs are merged without considering if they both are dirty or
>>> not (34228d473efe). To monitor changes over the memory, sometimes VMAs are
>>> split to clear the soft-dirty bit in the VMA flags. But sometimes kernel
>>> decide to merge them backup. It is so waste of resources.
>>
>> Maybe you'd want a per-process option to not merge if the VM_SOFTDIRTY
>> property differs. But that might be just one alternative for handling this
>> case.
>>
>>>
>>> To keep things consistent, the default behavior of the IOCTL is to output
>>> even the extra non-dirty pages as dirty from the kernel noise. A optional
>>> PAGEMAP_NO_REUSED_REGIONS flag is added for those use cases which aren't
>>> tolerant of extra non-dirty pages. This flag can be considered as something
>>> which is by-passing the already present buggy implementation in the kernel.
>>> It is not buggy per say as the issue can be solved if we don't allow the
>>> two VMA which have different soft-dirty bits to get merged. But we are
>>> allowing that so that the total number of VMAs doesn't increase. This was
>>> acceptable at the time, but now with the use case of monitoring a part of
>>> memory for soft-dirty doesn't want this merging. So either we need to
>>> revert 34228d473efe and PAGEMAP_NO_REUSED_REGIONS flag will not be needed
>>> or we should allow PAGEMAP_NO_REUSED_REGIONS or similar mechanism to ignore
>>> the extra dirty pages which aren't dirty in reality.
>>>
>>> When PAGEMAP_NO_REUSED_REGIONS flag is used, only the PTEs are checked to
>>> find if the pages are dirty. So re-used regions cannot be detected. This
>>> has the only side-effect of not checking the VMAs. So this is limitation of
>>> using this flag which should be acceptable in the current state of code.
>>> This limitation is okay for the users as they can clear the soft-dirty bit
>>> of the VMA before starting to monitor a range of memory for soft-dirtiness.
>>>
>>>
>>>> Please separate that part out from the other changes; I am still not
>>>> convinced that we want this and what the semantical implications are.
>>>>
>>>> Let's take a look at an example: can_change_pte_writable()
>>>>
>>>>      /* Do we need write faults for softdirty tracking? */
>>>>      if (vma_soft_dirty_enabled(vma) && !pte_soft_dirty(pte))
>>>>          return false;
>>>>
>>>> We care about PTE softdirty tracking, if it is enabled for the VMA.
>>>> Tracking is enabled if: vma_soft_dirty_enabled()
>>>>
>>>>      /*
>>>>       * Soft-dirty is kind of special: its tracking is enabled when
>>>>       * the vma flags not set.
>>>>       */
>>>>      return !(vma->vm_flags & VM_SOFTDIRTY);
>>>>
>>>> Consequently, if VM_SOFTDIRTY is set, we are not considering the soft_dirty
>>>> PTE bits accordingly.
>>> Sorry, I'm unable to completely grasp the meaning of the example. We have
>>> followed clear_refs_write() to write the soft-dirty bit clearing code in
>>> the current patch. Dirtiness of the VMA and the PTE may be set
>>> independently. Newer allocated memory has dirty bit set in the VMA. When
>>> something is written the memory, the soft dirty bit is set in the PTEs as
>>> well regardless if the soft dirty bit is set in the VMA or not.
>>>
>>
>> Let me try to find a simple explanation:
>>
>> After clearing a SOFTDIRTY PTE flag inside an area with VM_SOFTDIRTY set,
>> there are ways that PTE could get written to and it could become dirty,
>> without the PTE becoming softdirty.
>>
>> Essentially, inside a VMA with VM_SOFTDIRTY set, the PTE softdirty values
>> might be stale: there might be entries that are softdirty even though the
>> PTE is *not* marked softdirty.
> Can someone please share the example to reproduce this? In all of my
> testing, even if I ignore VM_SOFTDIRTY and only base my decision of
> soft-dirtiness on individual pages, it always passes.

Quick reproducer (the first and easiest one that triggered :) )
attached.

With no kernel changes, it works as expected.

# ./softdirty_mprotect

With the following kernel change to simulate what you propose it fails:

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index d22687d2e81e..f2c682bf7f64 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -1457,8 +1457,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
flags |= PM_FILE;
if (page && !migration && page_mapcount(page) == 1)
flags |= PM_MMAP_EXCLUSIVE;
- if (vma->vm_flags & VM_SOFTDIRTY)
- flags |= PM_SOFT_DIRTY;
+ //if (vma->vm_flags & VM_SOFTDIRTY)
+ // flags |= PM_SOFT_DIRTY;

return make_pme(frame, flags);
}

# ./softdirty_mprotect
Page #1 should be softdirty

--
Thanks,

David / dhildenb

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#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdint.h>
#include <stdbool.h>
#include <sys/mman.h>

static size_t pagesize;
static int pagemap_fd;

static void clear_softdirty(void)
{
int fd = open("/proc/self/clear_refs", O_WRONLY);
const char *ctrl = "4";
int ret;

if (fd < 0) {
fprintf(stderr, "open() failed\n");
exit(1);
}
ret = write(fd, ctrl, strlen(ctrl));
close(fd);
if (ret != strlen(ctrl)) {
fprintf(stderr, "write() failed\n");
exit(1);
}
}

static uint64_t pagemap_get_entry(int fd, char *start)
{
const unsigned long pfn = (unsigned long)start / pagesize;
uint64_t entry;
int ret;

ret = pread(fd, &entry, sizeof(entry), pfn * sizeof(entry));
if (ret != sizeof(entry)) {
fprintf(stderr, "pread() failed\n");
exit(1);
}

return entry;
}

static bool pagemap_is_softdirty(int fd, char *start)
{
uint64_t entry = pagemap_get_entry(fd, start);

return entry & 0x0080000000000000ull;
}

void main(void)
{
char *mem, *mem2;

pagesize = getpagesize();
pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
if (pagemap_fd < 0) {
fprintf(stderr, "open() failed\n");
exit(1);
}

/* Map 2 pages. */
mem = mmap(0, 2 * pagesize, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANON, -1, 0);
if (mem == MAP_FAILED) {
fprintf(stderr, "mmap() failed\n");
exit(1);
}

/* Populate both pages. */
memset(mem, 1, 2 * pagesize);

if (!pagemap_is_softdirty(pagemap_fd, mem))
fprintf(stderr, "Page #1 should be softdirty\n");
if (!pagemap_is_softdirty(pagemap_fd, mem + pagesize))
fprintf(stderr, "Page #2 should be softdirty\n");

/*
* Start softdirty tracking. Clear VM_SOFTDIRTY and clear the softdirty
* PTE bit.
*/
clear_softdirty();

if (pagemap_is_softdirty(pagemap_fd, mem))
fprintf(stderr, "Page #1 should not be softdirty\n");
if (pagemap_is_softdirty(pagemap_fd, mem + pagesize))
fprintf(stderr, "Page #2 should not be softdirty\n");

/*
* Remap the second page. The VMA gets VM_SOFTDIRTY set. Both VMAs
* get merged such that the resulting VMA has VM_SOFTDIRTY set.
*/
mem2 = mmap(mem + pagesize, pagesize, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANON|MAP_FIXED, -1, 0);
if (mem2 == MAP_FAILED) {
fprintf(stderr, "mmap() failed\n");
exit(1);
}

/* Protect + unprotect. */
mprotect(mem, 2 * pagesize, PROT_READ);
mprotect(mem, 2 * pagesize, PROT_READ|PROT_WRITE);

/* Modify both pages. */
memset(mem, 2, 2 * pagesize);

if (!pagemap_is_softdirty(pagemap_fd, mem))
fprintf(stderr, "Page #1 should be softdirty\n");
if (!pagemap_is_softdirty(pagemap_fd, mem + pagesize))
fprintf(stderr, "Page #2 should be softdirty\n");
}