Re: [PATCH] mm: set the vma flags dirty before testing if it is mergeable

[David Hildenbrand]

On 22.11.22 09:24, Muhammad Usama Anjum wrote:
> The VM_SOFTDIRTY should be set in the vma flags to be tested if new
> allocation should be merged in previous vma or not. With this patch,
> the new allocations are merged in the previous VMAs.
>
> I've tested it by reverting the commit 34228d473efe ("mm: ignore
> VM_SOFTDIRTY on VMA merging") and after adding this following patch,
> I'm seeing that all the new allocations done through mmap() are merged
> in the previous VMAs. The number of VMAs doesn't increase drastically
> which had contributed to the crash of gimp. If I run the same test after
> reverting and not including this patch, the number of VMAs keep on
> increasing with every mmap() syscall which proves this patch.
>
> The commit 34228d473efe ("mm: ignore VM_SOFTDIRTY on VMA merging")
> seems like a workaround. But it lets the soft-dirty and non-soft-dirty
> VMA to get merged. It helps in avoiding the creation of too many VMAs.
> But it creates the problem while adding the feature of clearing the
> soft-dirty status of only a part of the memory region.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: d9104d1ca966 ("mm: track vma changes with VM_SOFTDIRTY bit")
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> ---
> We need more testing of this patch.
>
> While implementing clear soft-dirty bit for a range of address space, I'm
> facing an issue. The non-soft dirty VMA gets merged sometimes with the soft
> dirty VMA. Thus the non-soft dirty VMA become dirty which is undesirable.
> When discussed with the some other developers they consider it the
> regression. Why the non-soft dirty page should appear as soft dirty when it
> isn't soft dirty in reality? I agree with them. Should we revert
> 34228d473efe or find a workaround in the IOCTL?
>
> * Revert may cause the VMAs to expand in uncontrollable situation where the
> soft dirty bit of a lot of memory regions or the whole address space is
> being cleared again and again. AFAIK normal process must either be only
> clearing a few memory regions. So the applications should be okay. There is
> still chance of regressions if some applications are already using the
> soft-dirty bit. I'm not sure how to test it.
>
> * Add a flag in the IOCTL to ignore the dirtiness of VMA. The user will
> surely lose the functionality to detect reused memory regions. But the
> extraneous soft-dirty pages would not appear. I'm trying to do this in the
> patch series [1]. Some discussion is going on that this fails with some
> mprotect use case [2]. I still need to have a look at the mprotect selftest
> to see how and why this fails. I think this can be implemented after some
> more work probably in mprotect side.
>
> [1] https://lore.kernel.org/all/20221109102303.851281-1-usama.anjum@xxxxxxxxxxxxx/
> [2] https://lore.kernel.org/all/bfcae708-db21-04b4-0bbe-712badd03071@xxxxxxxxxx/
> ---
>   mm/mmap.c | 21 +++++++++++----------
>   1 file changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f9b96b387a6f..6934b8f61fdc 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1708,6 +1708,15 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>   		vm_flags |= VM_ACCOUNT;
>   	}
>   
> +	/*
> +	 * New (or expanded) vma always get soft dirty status.
> +	 * Otherwise user-space soft-dirty page tracker won't
> +	 * be able to distinguish situation when vma area unmapped,
> +	 * then new mapped in-place (which must be aimed as
> +	 * a completely new data area).
> +	 */
> +	vm_flags |= VM_SOFTDIRTY;
> +
>   	/*
>   	 * Can we just expand an old mapping?
>   	 */
> @@ -1823,15 +1832,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>   	if (file)
>   		uprobe_mmap(vma);
>   
> -	/*
> -	 * New (or expanded) vma always get soft dirty status.
> -	 * Otherwise user-space soft-dirty page tracker won't
> -	 * be able to distinguish situation when vma area unmapped,
> -	 * then new mapped in-place (which must be aimed as
> -	 * a completely new data area).
> -	 */
> -	vma->vm_flags |= VM_SOFTDIRTY;
> -
>   	vma_set_page_prot(vma);

vma_set_page_prot(vma) has to be called after adjusting vma->vm_flags.

Did not look into the details here, but that jumped at me.

--
Thanks,

David / dhildenb