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Date:   Thu, 23 Mar 2023 21:31:40 +0800
From:   Baoquan He <bhe@redhat.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Lorenzo Stoakes <lstoakes@gmail.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Uladzislau Rezki <urezki@gmail.com>,
        Matthew Wilcox <willy@infradead.org>,
        Liu Shixin <liushixin2@huawei.com>,
        Jiri Olsa <jolsa@kernel.org>, Jens Axboe <axboe@kernel.dk>,
        Alexander Viro <viro@zeniv.linux.org.uk>
Subject: Re: [PATCH v7 4/4] mm: vmalloc: convert vread() to vread_iter()
Message-ID: <ZBxUvBFHcQvsl0r9@MiWiFi-R3L-srv>
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On 03/23/23 at 11:38am, David Hildenbrand wrote:
> On 23.03.23 11:36, Baoquan He wrote:
> > On 03/23/23 at 06:44am, Lorenzo Stoakes wrote:
> > > On Thu, Mar 23, 2023 at 10:52:09AM +0800, Baoquan He wrote:
> > > > 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.
> > > 
> > > This is refererrring to what's happening in kcore.c, not vread_iter(),
> > > i.e. the looped read/faultin.
> > > 
> > > The reason we bail out if failt_in_iov_iter_writeable() is that would
> > > indicate an error had occurred.
> > > 
> > > The whole point is to _optimistically_ try to perform the operation
> > > assuming the pages are faulted in. Ultimately we fault in via
> > > copy_to_user_nofault() which will either copy data or fail if the pages are
> > > not faulted in (will discuss this below a bit more in response to your
> > > other point).
> > > 
> > > If this fails, then we fault in, and try again. We loop because there could
> > > be some extremely unfortunate timing with a race on e.g. swapping out or
> > > migrating pages between faulting in and trying to write out again.
> > > 
> > > This is extremely unlikely, but to avoid any chance of breaking userland we
> > > repeat the operation until it completes. In nearly all real-world
> > > situations it'll either work immediately or loop once.
> > 
> > Thanks a lot for these helpful details with patience. I got it now. I was
> > mainly confused by the while(true) loop in KCORE_VMALLOC case of read_kcore_iter.
> > 
> > Now is there any chance that the faulted in memory is swapped out or
> > migrated again before vread_iter()? fault_in_iov_iter_writeable() will
> > pin the memory? I didn't find it from code and document. Seems it only
> > falults in memory. If yes, there's window between faluting in and
> > copy_to_user_nofault().
> > 
> 
> See the documentation of fault_in_safe_writeable():
> 
> "Note that we don't pin or otherwise hold the pages referenced that we fault
> in.  There's no guarantee that they'll stay in memory for any duration of
> time."

Thanks for the info. Then swapping out/migration could happen again, so
that's why while(true) loop is meaningful.