Re: [PATCH v7 4/4] mm: vmalloc: convert vread() to vread_iter()

Baoquan He
2023-03-23T02:52:22Z

On 03/22/23 at 06:57pm, 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, and try to write to user memory optimistically but
> with faults disabled via copy_page_to_iter_nofault(). We already have
> preemption disabled by holding a spin lock. We continue faulting in until
> the operation is complete.

I don't understand the sentences here. In vread_iter(), the actual
content reading is done in aligned_vread_iter(), otherwise we zero
filling the region. In aligned_vread_iter(), we will use
vmalloc_to_page() to get the mapped page and read out, otherwise zero
fill. While in this patch, fault_in_iov_iter_writeable() fault in memory
of iter one time and will bail out if failed. I am wondering why we
continue faulting in until the operation is complete, and how that is done.

If we look into the failing point in vread_iter(), it's mainly coming
from copy_page_to_iter_nofault(), e.g page_copy_sane() checking failed,
i->data_source checking failed. If these conditional checking failed,
should we continue reading again and again? And this is not related to
memory faulting in. I saw your discussion with David, but I am still a
little lost. Hope I can learn it, thanks in advance.

......
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 08b795fd80b4..25b44b303b35 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
......
> @@ -507,13 +503,30 @@ 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) {
> - ret = -EFAULT;
> - goto out;
> + {
> + const char *src = (char *)start;
> + size_t read = 0, left = tsz;
> +
> + /*
> + * vmalloc uses spinlocks, so we optimistically try to
> + * read memory. If this fails, fault pages in and try
> + * again until we are done.
> + */
> + while (true) {
> + read += vread_iter(iter, src, left);
> + if (read == tsz)
> + break;
> +
> + src += read;
> + left -= read;
> +
> + if (fault_in_iov_iter_writeable(iter, left)) {
> + ret = -EFAULT;
> + goto out;
> + }
> }
> break;
> + }
> case KCORE_USER:
> /* User page is handled prior to normal kernel page: */
> if (copy_to_iter((char *)start, tsz, iter) != tsz) {