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

[David Hildenbrand]

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,

David / dhildenb