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SubjectRe: [PATCH v4 4/4] mm: vmalloc: convert vread() to vread_iter()
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On 21.03.23 21:54, Lorenzo Stoakes wrote:
> Having previously laid the foundation for converting vread() to an iterator
> function, pull the trigger and do so.
>
> This patch attempts to provide minimal refactoring and to reflect the
> existing logic as best we can, for example we continue to zero portions of
> memory not read, as before.
>
> Overall, there should be no functional difference other than a performance
> improvement in /proc/kcore access to vmalloc regions.
>
> Now we have eliminated the need for a bounce buffer in read_kcore_iter(),
> we dispense with it. We need to ensure userland pages are faulted in before
> proceeding, as we take spin locks.
>
> Additionally, we must account for the fact that at any point a copy may
> fail if this happens, we exit indicating fewer bytes retrieved than
> expected.
>
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> ---
> fs/proc/kcore.c | 26 ++---
> include/linux/vmalloc.h | 3 +-
> mm/nommu.c | 10 +-
> mm/vmalloc.c | 234 +++++++++++++++++++++++++---------------
> 4 files changed, 160 insertions(+), 113 deletions(-)
>
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 25e0eeb8d498..221e16f75ba5 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -307,13 +307,9 @@ static void append_kcore_note(char *notes, size_t *i, const char *name,
> *i = ALIGN(*i + descsz, 4);
> }
>
> -static ssize_t
> -read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
> +static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
> {
> - struct file *file = iocb->ki_filp;
> - char *buf = file->private_data;
> loff_t *ppos = &iocb->ki_pos;
> -
> size_t phdrs_offset, notes_offset, data_offset;
> size_t page_offline_frozen = 1;
> size_t phdrs_len, notes_len;
> @@ -507,9 +503,12 @@ read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
>
> switch (m->type) {
> case KCORE_VMALLOC:
> - vread(buf, (char *)start, tsz);
> - /* we have to zero-fill user buffer even if no read */
> - if (copy_to_iter(buf, tsz, iter) != tsz) {
> + /*
> + * Make sure user pages are faulted in as we acquire
> + * spinlocks in vread_iter().
> + */
> + if (fault_in_iov_iter_writeable(iter, tsz) ||
> + vread_iter(iter, (char *)start, tsz) != tsz) {
> ret = -EFAULT;
> goto out;
> }

What if we race with swapout after faulting the pages in? Or some other
mechanism to write-protect the user space pages?

Also, "This is primarily useful when we already know that some or all of
the pages in @i aren't in memory". This order of events might slow down
things quite a bit if I am not wrong.


Wouldn't you want to have something like:

while (vread_iter(iter, (char *)start, tsz) != tsz) {
if (fault_in_iov_iter_writeable(iter, tsz)) {
ret = -EFAULT;
goto out;
}
}

Or am I missing something?

--
Thanks,

David / dhildenb

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