On Tue, Jun 17, 2014 at 1:31 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote: > On Tue, 17 Jun 2014, Andy Lutomirski wrote: >> On Tue, Jun 17, 2014 at 9:51 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote: >> > 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: >> >>> 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. > > I do agree with you, Andy. > > I agree with David that a fallocate (of the fill-in-holes variety) > does not have to be prohibited on a sealed file, that detection of > holes is not an issue with respect to sealing, and that fallocate > by the recipient could be used to "post-seal" the object to safety. > > But it doesn't feel right, and we shall be re-explaining and apologizing > for it for months to come, until we just fix it. I suspect David didn't > want to add a dependency upon me to fix it, and I didn't want to be > rushed into fixing it (nor is it a job I'd be comfortable to delegate). I suppose it would be possible to merge memfd_create as is, and then to fix the zero page thing and make fallocate on a SEAL_WRITEd file be a no-op. It would be silly for code to fallocate actual sealed-with-holes files and allocate fresh pages that are guaranteed to only ever contain zeros. --Andy -- 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
