On Fri, Jun 1, 2018 at 8:01 AM, Gerald Schaefer <gerald.schaefer@xxxxxxxxxx> wrote: > On Tue, 29 May 2018 13:26:33 -0700 > Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > >> On Wed, May 23, 2018 at 11:50 AM, Gerald Schaefer >> <gerald.schaefer@xxxxxxxxxx> wrote: >> > On Tue, 22 May 2018 08:28:06 +0200 >> > Christoph Hellwig <hch@xxxxxx> wrote: >> > >> >> On Mon, May 21, 2018 at 11:04:10AM +0200, Jan Kara wrote: >> >> > We definitely do have customers using "execute in place" on s390x from >> >> > dcssblk. I've got about two bug reports for it when customers were updating >> >> > from old kernels using original XIP to kernels using DAX. So we need to >> >> > keep that working. >> >> >> >> That is all good an fine, but I think time has come where s390 needs >> >> to migrate to provide the pmem API so that we can get rid of these >> >> special cases. Especially given that the old XIP/legacy DAX has all >> >> kinds of known bugs at this point in time. >> > >> > I haven't yet looked at this patch series, but I can feel that this >> > FS_DAX_LIMITED workaround is beginning to cause some headaches, apart >> > from being quite ugly of course. >> > >> > Just to make sure I still understand the basic problem, which I thought >> > was missing struct pages for the dcssblk memory, what exactly do you >> > mean with "provide the pmem API", is there more to do? >> >> No, just 'struct page' is needed. >> >> What used to be the pmem API is now pushed down into to dax_operations >> provided by the device driver. dcssblk is free to just redirect to the >> generic implementations for copy_from_iter() and copy_to_iter(), and >> be done. I.e. we've removed the "pmem API" requirement. >> >> > I do have a prototype patch lying around that adds struct pages, but >> > didn't yet have time to fully test/complete it. Of course we initially >> > introduced XIP as a mechanism to reduce memory consumption, and that >> > is probably the use case for the remaining customer(s). Adding struct >> > pages would somehow reduce that benefit, but as long as we can still >> > "execute in place", I guess it will be OK. >> >> The pmem driver has the option to allocate the 'struct page' map out >> of pmem directly. If the overhead of having the map in System RAM is >> too high it could borrow the same approach, but that adds another >> degree of configuration complexity freedom. >> > > Thanks for clarifying, and mentioning the pmem altmap support, that > looks interesting. I also noticed that I probably should enable > CONFIG_ZONE_DEVICE for s390, and use devm_memremap_pages() to get > the struct pages, rather than my homegrown solution so far. This will > take some time however, so I hope you can live with the FS_DAX_LIMITED > a little longer. > Yes, no urgent rush as far as I can see. Thanks Gerald.
