Re: can buffer cache pages be used in ext_pgs mbufs?

Konstantin Belousov wrote:
>On Tue, Aug 11, 2020 at 03:10:39AM +0000, Rick Macklem wrote:
>> Konstantin Belousov wrote:
>> >On Mon, Aug 10, 2020 at 12:46:00AM +0000, Rick Macklem wrote:
>> >> Konstantin Belousov wrote:
>> >> >On Fri, Aug 07, 2020 at 09:43:14PM -0700, Kirk McKusick wrote:
>> >> >> I do not have the answer to your question, but I am copying Kostik
>> >> >> as if anyone knows the answer, it is probably him.
>> >> >>
>> >> >>       ~Kirk
>> >> >>
>> >> >> =-=-=
>> >> >I do not know the exact answer, this is why I did not followed up on the
>> >> >original question on current_at_.  In particular, I have no idea about the
>> >> >ext_pgs mechanism.
>> >> >
>> >> >Still I can point one semi-obvious aspect of your proposal.
>> >> >
>> >> >When the buffer is written (with bwrite()), its pages are sbusied and
>> >> >the write mappings of them are invalidated. The end effect is that no
>> >> >modifications to the pages are possible until they are unbusied. This,
>> >> >together with the lock of the buffer that holds the pages, effectively
>> >> >stops all writes either through write(2) or by mmaped regions.
>> >> >
>> >> >In other words, any access for write to the range of file designated by
>> >> >the buffer, causes the thread to block until the pages are unbusied and
>> >> >the buffer is unlocked.  Which in described case would mean, until NFS
>> >> >server responds.
>> >> >
>> >> >If this is fine, then ok.
>> >> For what I am thinking of, I would say that is fine, since the ktls code reads
>> >> the pages to encrypt/send them, but can use other allocated pages for
>> >> the encrypted data.
>> >>
>> >> >Rick, do you know anything about the vm page lifecycle as mb_ext_pgs ?
>> >> Well, the anonymous pages (the only ones I've been using sofar) are
>> >> allocated with:
>> >>         vm_page_alloc(NULL, 0, VM_ALLOC_NORMAL | VM_ALLOC_NOOBJ |
>> >>                VM_ALLOC_NODUMP | VM_ALLOC_WIRED);
>> >>
>> >> and then the m_ext_ext_free function (mb_free_mext_pgs()) does:
>> >>         vm_page_unwire_noq(pg);
>> >>         vm_page_free(pg);
>> >> on each of them.
>> >>
>> >> m->m_ext_ext_free() is called in tls_encrypt() when it no longer wants the
>> >> pages, but is normally called via m_free(m), which calls mb_free_extpg(m),
>> >> although there are a few other places.
>> >>
>> >> Since m_ext_ext_free is whatever function you want to make it, I suppose the
>> >> answer is "until your m_ext.ext_free" function is called.
>> >>
>> >> At this time, for ktls, if you are using software encryption, the call to ktls_encrypt(),
>> >> which is done before passing the mbufs down to TCP is when it is done with the
>> >> unencrypted data pages. (I suppose there is no absolute guarantee that this
>> >> happens before the kernel RPC layer times out waiting for an RPC reply, but it
>> >> is almost inconceivable, since this happens before the RPC request is passed
>> >> down to TCP.)
>> >>
>> >> The case I now think is more problematic is the "hardware assist" case. Although
>> >> no hardware/driver yet does this afaik, I suspect that the unencrypted data page
>> >> mbufs could end up stuck in TCP for a long time, in case a retransmit is needed.
>> >>
>> >> So, I now think I might need to delay the bufdone() call until the m_ext_ext_free()
>> >> call has been done for the pages, if they are buffer cache pages?
>> >> --> Usually I would expect the m_ext_ext_free() call for the mbuf(s) that
>> >>        hold the data to be written to the server to be done long before
>> >>        bufdone() would be called for the buffer that is being written,
>> >>        but there is no guarantee.
>> >>
>> >> Am I correct in assuming that the pages for the buffer will remain valid and
>> >> readable through the direct map until bufdone() is called?
>> >> If I am correct w.r.t. this, it should work so long as the m_ext_ext_free() calls
>> >> for the pages happen before the bufdone() call on the bp, I think?
>> >
>> >I think there is further complication with non-anonymous pages.
>> >You want (or perhaps need) the page content to be immutable and not
>> >changed while you pass pages around and give the for ktls sw or hw
>> >processing.  Otherwise it could not pass the TLS authentification if
>> >page was changed in process.
>> >
>> >Similar issue exists when normal buffer writes are scheduled through
>> >the strategy(), and you can see that bufwrite() does vfs_busy_pages()
>> >with clear_modify=1, which does two things:
>> >- sbusy the pages (sbusy pages can get new read-only mappings, but cannot
>> >  be mapped rw)
>> >- pmap_remove_write() on the pages to invalidate all current writeable
>> >  mappings.
>> >
>> >This state should be kept until ktls is completely done with the pages.
>> I am now thinking that this is done exactly as you describe above and
>> doesn't require any changes.
>>
>> The change I am planning is below the strategy routine in the function
>> that does the write RPC.
>> It currently copies the data from the buffer into mbuf clusters.
>> After this change, it would put the physical page #s for the buffer in the
>> mbuf(s) and then wait for them all to be m_ext_ext_free()d before calling
>> bufdone().
>> --> The only difference is the wait before the bufdone() call in the RPC layer
>>        below the strategy routine. (bufdone() is the only call the NFS client
>>        seems to do below the strategy routine, so I assume it ends the state
>>        you describe above?)
>>
>As far as pages are put into mbuf clusters only after bwrite() that
>did vfs_busy_pages(), and bufdone() is called not earlier than network
>finished with the mbufs, it should be ok.
I've coded it up and, at least for a little testing sofar, it seems to work ok.

Thanks for your comments, rick