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SubjectRe: [PATCH v4 1/8] mm: pass task to do_madvise
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On Wed, 2020-02-12 at 15:39 -0800, Minchan Kim wrote:
> In upcoming patches, do_madvise will be called from external process
> context so it shouldn't asssume "current" is always hinted process's
> task_struct. Thus, let's get the mm_struct from vma->vm_mm, not
> current because vma is always hinted process's one. And let's pass
> *current* as new task argument of do_madvise so it shouldn't change
> existing behavior.
>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> fs/io_uring.c | 2 +-
> include/linux/mm.h | 3 ++-
> mm/madvise.c | 37 ++++++++++++++++++++-----------------
> 3 files changed, 23 insertions(+), 19 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 63beda9bafc5..6307206b970f 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2736,7 +2736,7 @@ static int io_madvise(struct io_kiocb *req, struct io_kiocb **nxt,
> if (force_nonblock)
> return -EAGAIN;
>
> - ret = do_madvise(ma->addr, ma->len, ma->advice);
> + ret = do_madvise(current, ma->addr, ma->len, ma->advice);
> if (ret < 0)
> req_set_fail_links(req);
> io_cqring_add_event(req, ret);
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 52269e56c514..8cb41131ec96 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2323,7 +2323,8 @@ extern int __do_munmap(struct mm_struct *, unsigned long, size_t,
> struct list_head *uf, bool downgrade);
> extern int do_munmap(struct mm_struct *, unsigned long, size_t,
> struct list_head *uf);
> -extern int do_madvise(unsigned long start, size_t len_in, int behavior);
> +extern int do_madvise(struct task_struct *task, unsigned long start,
> + size_t len_in, int behavior);
>
> static inline unsigned long
> do_mmap_pgoff(struct file *file, unsigned long addr,
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 43b47d3fae02..ab4011ba2d9e 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -256,6 +256,7 @@ static long madvise_willneed(struct vm_area_struct *vma,
> {
> struct file *file = vma->vm_file;
> loff_t offset;
> + struct mm_struct *mm = vma->vm_mm;
>
> *prev = vma;
> #ifdef CONFIG_SWAP

I would probably move the declaration of the mm variable to the top just
so you don't have the large "offset" valley between the two long variable
declarations.

> @@ -288,12 +289,12 @@ static long madvise_willneed(struct vm_area_struct *vma,
> */
> *prev = NULL; /* tell sys_madvise we drop mmap_sem */
> get_file(file);
> - up_read(&current->mm->mmap_sem);
> + up_read(&mm->mmap_sem);
> offset = (loff_t)(start - vma->vm_start)
> + ((loff_t)vma->vm_pgoff << PAGE_SHIFT);
> vfs_fadvise(file, offset, end - start, POSIX_FADV_WILLNEED);
> fput(file);
> - down_read(&current->mm->mmap_sem);
> + down_read(&mm->mmap_sem);
> return 0;
> }
>
> @@ -674,9 +675,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr,
> }
> out:
> if (nr_swap) {
> - if (current->mm == mm)
> - sync_mm_rss(mm);
> -
> + sync_mm_rss(mm);
> add_mm_counter(mm, MM_SWAPENTS, nr_swap);
> }
> arch_leave_lazy_mmu_mode();

This seems like it is taking things in the opposite direction of the other
changes. sync_mm_rss will operate on current if I am not mistaken. I don't
think you would want to add the stats from current to the stats of the
task you are updating.

It might make sense to add a new function that would allow you to sync the
remote task stats by creaing a version of sync_mm_rss that also takes a
task pointer.

> @@ -756,6 +755,7 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
> unsigned long start, unsigned long end,
> int behavior)
> {
> + struct mm_struct *mm = vma->vm_mm;
> *prev = vma;
> if (!can_madv_lru_vma(vma))
> return -EINVAL;
> @@ -763,8 +763,8 @@ static long madvise_dontneed_free(struct vm_area_struct *vma,
> if (!userfaultfd_remove(vma, start, end)) {
> *prev = NULL; /* mmap_sem has been dropped, prev is stale */
>
> - down_read(&current->mm->mmap_sem);
> - vma = find_vma(current->mm, start);
> + down_read(&mm->mmap_sem);
> + vma = find_vma(mm, start);
> if (!vma)
> return -ENOMEM;
> if (start < vma->vm_start) {

This piece of code has me wondering if it is valid to be using vma->mm at
the start of the function. I assume we are probably safe since we read the
mm value before the semaphore was released in userfaultfd_remove. It might
make more sense to just pass the task to the function and use task->mm-
>mmap_sem instead.

It might be simpler, safer, and easier to review to just go through and
add the task struct as needed and then simply replace references to
current->mm with task->mm.

> @@ -818,6 +818,7 @@ static long madvise_remove(struct vm_area_struct *vma,
> loff_t offset;
> int error;
> struct file *f;
> + struct mm_struct *mm = vma->vm_mm;
>
> *prev = NULL; /* tell sys_madvise we drop mmap_sem */
>
> @@ -845,13 +846,13 @@ static long madvise_remove(struct vm_area_struct *vma,
> get_file(f);
> if (userfaultfd_remove(vma, start, end)) {
> /* mmap_sem was not released by userfaultfd_remove() */
> - up_read(&current->mm->mmap_sem);
> + up_read(&mm->mmap_sem);
> }
> error = vfs_fallocate(f,
> FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
> offset, end - start);
> fput(f);
> - down_read(&current->mm->mmap_sem);
> + down_read(&mm->mmap_sem);
> return error;
> }
>
> @@ -1044,7 +1045,8 @@ madvise_behavior_valid(int behavior)
> * -EBADF - map exists, but area maps something that isn't a file.
> * -EAGAIN - a kernel resource was temporarily unavailable.
> */
> -int do_madvise(unsigned long start, size_t len_in, int behavior)
> +int do_madvise(struct task_struct *task, unsigned long start,
> + size_t len_in, int behavior)
> {
> unsigned long end, tmp;
> struct vm_area_struct *vma, *prev;
> @@ -1053,6 +1055,7 @@ int do_madvise(unsigned long start, size_t len_in, int behavior)
> int write;
> size_t len;
> struct blk_plug plug;
> + struct mm_struct *mm = task->mm;
>
> start = untagged_addr(start);
>
> @@ -1082,10 +1085,10 @@ int do_madvise(unsigned long start, size_t len_in, int behavior)
>
> write = madvise_need_mmap_write(behavior);
> if (write) {
> - if (down_write_killable(&current->mm->mmap_sem))
> + if (down_write_killable(&mm->mmap_sem))
> return -EINTR;
> } else {
> - down_read(&current->mm->mmap_sem);
> + down_read(&mm->mmap_sem);
> }
>
> /*
> @@ -1093,7 +1096,7 @@ int do_madvise(unsigned long start, size_t len_in, int behavior)
> * ranges, just ignore them, but return -ENOMEM at the end.
> * - different from the way of handling in mlock etc.
> */
> - vma = find_vma_prev(current->mm, start, &prev);
> + vma = find_vma_prev(mm, start, &prev);
> if (vma && start > vma->vm_start)
> prev = vma;
>
> @@ -1130,19 +1133,19 @@ int do_madvise(unsigned long start, size_t len_in, int behavior)
> if (prev)
> vma = prev->vm_next;
> else /* madvise_remove dropped mmap_sem */
> - vma = find_vma(current->mm, start);
> + vma = find_vma(mm, start);
> }
> out:
> blk_finish_plug(&plug);
> if (write)
> - up_write(&current->mm->mmap_sem);
> + up_write(&mm->mmap_sem);
> else
> - up_read(&current->mm->mmap_sem);
> + up_read(&mm->mmap_sem);
>
> return error;
> }
>
> SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
> {
> - return do_madvise(start, len_in, behavior);
> + return do_madvise(current, start, len_in, behavior);
> }


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