The vma_merge() interface is very confusing and its implementation has led to numerous bugs as a result of that confusion. In addition there is duplication both in invocation of vma_merge(), but also in the common mprotect()-style pattern of attempting a merge, then if this fails, splitting the portion of a VMA about to have its attributes changed. This pattern has been copy/pasted around the kernel in each instance where such an operation has been required, each very slightly modified from the last to make it even harder to decipher what is going on. Simplify the whole thing by dividing the actual uses of vma_merge() and split_vma() into specific and abstracted functions and de-duplicate the vma_merge()/split_vma() pattern altogether. Doing so also opens the door to changing how vma_merge() is implemented - by knowing precisely what cases a caller is invoking rather than having a central interface where anything might happen we can untangle the brittle and confusing vma_merge() implementation into something more workable. For mprotect()-like cases we introduce vma_modify() which performs the vma_merge()/split_vma() pattern, returning a pointer to either the merged or split VMA or an ERR_PTR(err) if the splits fail. We provide a number of inline helper functions to make things even clearer:- * vma_modify_flags() - Prepare to modify the VMA's flags. * vma_modify_flags_name() - Prepare to modify the VMA's flags/anon_vma_name * vma_modify_policy() - Prepare to modify the VMA's mempolicy. * vma_modify_flags_uffd() - Prepare to modify the VMA's flags/uffd context. For cases where a new VMA is attempted to be merged with adjacent VMAs we add:- * vma_merge_new_vma() - Prepare to merge a new VMA. * vma_merge_extend() - Prepare to extend the end of a new VMA. v4: * Correct bug where PTR_ERR() was accidentally pased prev rather than VMA, as suggested by Vlastimil. * Updated comment and styling, and moved from using pgoff in vma_merge_new_vma() to vma->vm_pgoff in case driver changes it, as suggested by Liam. v3: * Drop unnecessary VM_WARN_ON(). * Implement excellent suggestion from Vlastimil to simply have vma_modify() return the vma if merge fails (and no error occurs on split), as all callers really only need to deal with either the merged VMA or the original one if split. This simplifies things even further. https://lore.kernel.org/all/cover.1696929425.git.lstoakes@xxxxxxxxx/ v2: * Correct mistake where error cases would have been treated as success as pointed out by Vlastimil. * Move vma_policy() define to mm_types.h. * Move anon_vma_name(), anon_vma_name_alloc() and anon_vma_name_free() to mm_types.h from mm_inline.h. * These moves make it possible to implement the vma_modify_*() helpers as static inline declarations, so do so. * Spelling corrections and clarifications. https://lore.kernel.org/all/cover.1696884493.git.lstoakes@xxxxxxxxx/ v1: https://lore.kernel.org/all/cover.1696795837.git.lstoakes@xxxxxxxxx/ Lorenzo Stoakes (5): mm: move vma_policy() and anon_vma_name() decls to mm_types.h mm: abstract the vma_merge()/split_vma() pattern for mprotect() et al. mm: make vma_merge() and split_vma() internal mm: abstract merge for new VMAs into vma_merge_new_vma() mm: abstract VMA merge and extend into vma_merge_extend() helper fs/userfaultfd.c | 70 ++++++------------------ include/linux/mempolicy.h | 4 -- include/linux/mm.h | 69 ++++++++++++++++++++--- include/linux/mm_inline.h | 20 +------ include/linux/mm_types.h | 27 +++++++++ mm/internal.h | 7 +++ mm/madvise.c | 26 ++------- mm/mempolicy.c | 26 +-------- mm/mlock.c | 25 ++------- mm/mmap.c | 112 ++++++++++++++++++++++++++++++++------ mm/mprotect.c | 29 ++-------- mm/mremap.c | 30 +++++----- mm/nommu.c | 4 +- 13 files changed, 237 insertions(+), 212 deletions(-) -- 2.42.0
