On Fri 16-06-17 22:09:26, Ross Zwisler wrote: > On Thu, Jun 15, 2017 at 04:42:04PM +0200, Jan Kara wrote: > > On Wed 14-06-17 11:22:09, Ross Zwisler wrote: > > > To be able to use the common 4k zero page in DAX we need to have our PTE > > > fault path look more like our PMD fault path where a PTE entry can be > > > marked as dirty and writeable as it is first inserted, rather than waiting > > > for a follow-up dax_pfn_mkwrite() => finish_mkwrite_fault() call. > > > > > > Right now we can rely on having a dax_pfn_mkwrite() call because we can > > > distinguish between these two cases in do_wp_page(): > > > > > > case 1: 4k zero page => writable DAX storage > > > case 2: read-only DAX storage => writeable DAX storage > > > > > > This distinction is made by via vm_normal_page(). vm_normal_page() returns > > > false for the common 4k zero page, though, just as it does for DAX ptes. > > > Instead of special casing the DAX + 4k zero page case, we will simplify our > > > DAX PTE page fault sequence so that it matches our DAX PMD sequence, and > > > get rid of dax_pfn_mkwrite() completely. > > > > > > This means that insert_pfn() needs to follow the lead of insert_pfn_pmd() > > > and allow us to pass in a 'mkwrite' flag. If 'mkwrite' is set insert_pfn() > > > will do the work that was previously done by wp_page_reuse() as part of the > > > dax_pfn_mkwrite() call path. > > > > > > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> > > > > So I agree that getting rid of dax_pfn_mkwrite() and using fault handler in > > that case is a way to go. However I somewhat dislike the > > vm_insert_mixed_mkwrite() thing - it looks like a hack - and I'm aware that > > we have a similar thing for PMD which is ugly as well. Besides being ugly > > I'm also concerned that when 'mkwrite' is set, we just silently overwrite > > whatever PTE was installed at that position. Not that I'd see how that > > could screw us for DAX but still a concern that e.g. some PTE flag could > > get discarded by this is there... In fact, for !HAVE_PTE_SPECIAL > > architectures, you will leak zero page references by just overwriting the > > PTE - for those archs you really need to unmap zero page before replacing > > PTE (and the same for PMD I suppose). > > > > So how about some vmf_insert_pfn(vmf, pe_size, pfn) helper that would > > properly detect PTE / PMD case, read / write case etc., check that PTE did > > not change from orig_pte, and handle all the nasty details instead of > > messing with insert_pfn? > > I played around with this some today, and I wasn't super happy with the > results. Here were some issues I encountered: > > 1) The pte_mkyoung(), maybe_mkwrite() and pte_mkdirty() calls need to happen > with the PTE locked, and I'm currently able to piggy-back on the locking done > in insert_pfn(). If I keep those steps out of insert_pfn() I either have to > essentially duplicate all the work done by insert_pfn() into another function > so I can do everything I need under one lock, or I have to insert the PFN via > insert_pfn() (which as you point out, will just leave the pfn alone if it's > already present), then for writes I have to re-grab the PTE lock and set do > the mkwrite steps. > > Either of these work, but they both also seem kind of gross... > > 2) Combining the PTE and PMD cases into a common function will require > mm/memory.c to call vmf_insert_pfn_pmd(), which depends on > CONFIG_TRANSPARENT_HUGEPAGE being defined. This works, it just means some > more #ifdef CONFIG_TRANSPARENT_HUGEPAGE hackery in mm/memory.c. > > I agree that unconditionally overwriting the PTE when mkwrite is set is > undesireable, and should be fixed. My implementation of the wrapper just > didn't seem that natural, which usually tells me I'm headed down the wrong > path. Maybe I'm just not fully understanding what you intended? > > In any case, my current favorite soultion for this issue is still what I had > in v1: > > https://patchwork.kernel.org/patch/9772809/ > > with perhaps the removal of the new vm_insert_mixed_mkwrite() symbol, and just > adding a 'write' flag to vm_insert_mixed() and updating all the call sites, > and fixing the flow where mkwrite unconditionally overwrites the PTE? > > If not, can you help me understand what you think is ugly about the 'write' > flag to vm_insert_mixed() and vmf_insert_pfn_pmd()? Yeah, so write flag is probably OK. I just dislike the implicit "replace" side-effect of the write flag. If 'write' would just mean whether PTE is created writeable, that is fine with me. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
