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?
