On Tue, Jun 17, 2014 at 9:51 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote: > Hi > > On Tue, Jun 17, 2014 at 6:41 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >> On Tue, Jun 17, 2014 at 9:36 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote: >>> Hi >>> >>> On Tue, Jun 17, 2014 at 6:20 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: >>>> Can you summarize why holes can't be reliably backed by the zero page? >>> >>> To answer this, I will quote Hugh from "PATCH v2 1/3": >>> >>>> We do already use the ZERO_PAGE instead of allocating when it's a >>>> simple read; and on the face of it, we could extend that to mmap >>>> once the file is sealed. But I am rather afraid to do so - for >>>> many years there was an mmap /dev/zero case which did that, but >>>> it was an easily forgotten case which caught us out at least >>>> once, so I'm reluctant to reintroduce it now for sealing. >>>> >>>> Anyway, I don't expect you to resolve the issue of sealed holes: >>>> that's very much my territory, to give you support on. >>> >>> Holes can be avoided with a simple fallocate(). I don't understand why >>> I should make SEAL_WRITE do the fallocate for the caller. During the >>> discussion of memfd_create() I was told to drop the "size" parameter, >>> because it is redundant. I don't see how this implicit fallocate() >>> does not fall into the same category? >>> >> >> I'm really confused now. >> >> If I SEAL_WRITE a file, and then I mmap it PROT_READ, and then I read >> it, is that a "simple read"? If so, doesn't that mean that there's no >> problem? > > I assumed Hugh was talking about read(). So no, this is not about > memory-reads on mmap()ed regions. > > Looking at shmem_file_read_iter() I can see a ZERO_PAGE(0) call in > case shmem_getpage_gfp(SGP_READ) tells us there's a hole. I cannot see > anything like that in the mmap_region() and shmem_fault() paths. Would it be easy to fix this just for SEAL_WRITE files? Hugh? This would make the interface much nicer, IMO. --Andy > > Thanks > David -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
