Date: Tue, 11 Aug 2020 20:54:22 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Rick Macklem <rmacklem@uoguelph.ca> Cc: Kirk McKusick <mckusick@mckusick.com>, "freebsd-current@FreeBSD.org" <freebsd-current@freebsd.org> Subject: Re: can buffer cache pages be used in ext_pgs mbufs? Message-ID: <20200811175422.GP2551@kib.kiev.ua> In-Reply-To: <QB1PR01MB33643DFE1D132C76E33A9276DD450@QB1PR01MB3364.CANPRD01.PROD.OUTLOOK.COM> References: <202008080443.0784hEfh084650@chez.mckusick.com> <20200808144040.GD2551@kib.kiev.ua> <QB1PR01MB33646963EB1E38F2968A859BDD470@QB1PR01MB3364.CANPRD01.PROD.OUTLOOK.COM> <20200810170956.GL2551@kib.kiev.ua> <QB1PR01MB33643DFE1D132C76E33A9276DD450@QB1PR01MB3364.CANPRD01.PROD.OUTLOOK.COM>
next in thread | previous in thread | raw e-mail | index | archive | help
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@. 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.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20200811175422.GP2551>