linux-kernel - Re: [PATCH v2 2/4] mm: vmalloc: use rwsem, mutex for vmap_area_lock and vmap

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Date:   Sun, 19 Mar 2023 20:47:14 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Baoquan He <bhe@...hat.com>,
        Uladzislau Rezki <urezki@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v2 2/4] mm: vmalloc: use rwsem, mutex for vmap_area_lock
 and vmap_block->lock

On Sun, Mar 19, 2023 at 08:29:16PM +0000, Lorenzo Stoakes wrote:
> The basis for saying asynchronous was based on Documentation/filesystems/vfs.rst
> describing read_iter() as 'possibly asynchronous read with iov_iter as
> destination', and read_iter() is what is (now) invoked when accessing
> /proc/kcore.
> 
> However I agree this is vague and it is clearer to refer to the fact that we are
> now directly writing to user memory and thus wish to avoid spinlocks as we may
> need to fault in user memory in doing so.
> 
> Would it be ok for you to go ahead and replace that final paragraph with the
> below?:-
> 
> The reason for making this change is to build a basis for vread() to write
> to user memory directly via an iterator; as a result we may cause page
> faults during which we must not hold a spinlock. Doing this eliminates the
> need for a bounce buffer in read_kcore() and thus permits that to be
> converted to also use an iterator, as a read_iter() handler.

I'd say the purpose of the iterator is to abstract whether we're
accessing user memory, kernel memory or a pipe, so I'd suggest:

   The reason for making this change is to build a basis for vread() to
   write to memory via an iterator; as a result we may cause page faults
   during which we must not hold a spinlock. Doing this eliminates the
   need for a bounce buffer in read_kcore() and thus permits that to be
   converted to also use an iterator, as a read_iter() handler.

I'm still undecided whether this change is really a good thing.  I
think we have line-of-sight to making vmalloc (and thus kvmalloc)
usable from interrupt context, and this destroys that possibility.

I wonder if we can't do something like prefaulting the page before
taking the spinlock, then use copy_page_to_iter_atomic()