linux-kernel - Re: [PATCH v4 4/4] mm: vmalloc: convert vread() to vread

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Message-ID: <8ba0360e-57eb-93b0-3ae6-612f6b371bff@redhat.com>
Date:   Wed, 22 Mar 2023 12:18:08 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Baoquan He <bhe@...hat.com>, Uladzislau Rezki <urezki@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Liu Shixin <liushixin2@...wei.com>,
        Jiri Olsa <jolsa@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v4 4/4] mm: vmalloc: convert vread() to vread_iter()

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@...il.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