Re: [PATCH v8 2/3] mm: add a field to store names for private anonymous memory

[Cyrill Gorcunov]

On Fri, Aug 27, 2021 at 12:18:57PM -0700, Suren Baghdasaryan wrote:
> 
> The name is stored in a pointer in the shared union in vm_area_struct
> that points to a null terminated string. Anonymous vmas with the same
> name (equivalent strings) and are otherwise mergeable will be merged.
> The name pointers are not shared between vmas even if they contain the
> same name. The name pointer is stored in a union with fields that are
> only used on file-backed mappings, so it does not increase memory usage.
> 
> The patch is based on the original patch developed by Colin Cross, more
> specifically on its latest version [1] posted upstream by Sumit Semwal.
> It used a userspace pointer to store vma names. In that design, name
> pointers could be shared between vmas. However during the last upstreaming
> attempt, Kees Cook raised concerns [2] about this approach and suggested
> to copy the name into kernel memory space, perform validity checks [3]
> and store as a string referenced from vm_area_struct.
> One big concern is about fork() performance which would need to strdup
> anonymous vma names. Dave Hansen suggested experimenting with worst-case
> scenario of forking a process with 64k vmas having longest possible names
> [4]. I ran this experiment on an ARM64 Android device and recorded a
> worst-case regression of almost 40% when forking such a process. This
> regression is addressed in the followup patch which replaces the pointer
> to a name with a refcounted structure that allows sharing the name pointer
> between vmas of the same name. Instead of duplicating the string during
> fork() or when splitting a vma it increments the refcount.
> 
> [1] https://lore.kernel.org/linux-mm/20200901161459.11772-4-sumit.semwal@xxxxxxxxxx/
> [2] https://lore.kernel.org/linux-mm/202009031031.D32EF57ED@keescook/
> [3] https://lore.kernel.org/linux-mm/202009031022.3834F692@keescook/
> [4] https://lore.kernel.org/linux-mm/5d0358ab-8c47-2f5f-8e43-23b89d6a8e95@xxxxxxxxx/
...
> +
> +/* mmap_lock should be read-locked */
> +static inline bool is_same_vma_anon_name(struct vm_area_struct *vma,
> +					 const char *name)
> +{
> +	const char *vma_name = vma_anon_name(vma);
> +
> +	if (likely(!vma_name))
> +		return name == NULL;
> +
> +	return name && !strcmp(name, vma_name);
> +}

Hi Suren! There is very important moment with this new feature: if
we assign a name to some VMA it won't longer be mergeable even if
near VMA matches by all other attributes such as flags, permissions
and etc. I mean our vma_merge() start considering the vma namings
and names mismatch potentially blocks merging which happens now
without this new feature. Is it known behaviour or I miss something
pretty obvious here?