On Wed, 2008-01-16 at 06:48 +0100, Nick Piggin wrote: > On Tue, Jan 15, 2008 at 09:23:57PM -0800, Linus Torvalds wrote: > > On Tue, 15 Jan 2008, David Miller wrote: > > > > > > From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > > Date: Tue, 15 Jan 2008 20:48:42 -0800 (PST) > > > > > > > Can you give a pointer to some browsable archive? > > > > > > http://marc.info/?l=linux-arch > > > > .. and the discussion itself that actually explains why ARM has problems > > and why S390 suddenly _does_ have a bit for this after all? > > > > (Not that I consider marc to be really "browsable" in the first place.. ) > > > > Linus > > > I sent you an exact link to the thread on marc in an earlier message... > not many others archive linux-arch or linux-mm unfortunately. > > But no I didn't see a discussion of why s390 does have a bit, beyond the > s390 devs just asserting there is one, and providing a patch ;) I never > saw any of the discussions concluding that s390 does *not* have a bit > spare. Do you recall if they were public? Hmm, it all depends on the type of the pte. The hardware specs tell us that the last 8 bits of a pte is software defined. There is the hardware invalid bit 2**9 and the hardware read-only bit 2**10. Two of the software defined bits and the two hardware bits are used for the page type. The current code distinguishes 8 different types (the comments says six but that is wrong :-/): #define _PAGE_TYPE_EMPTY 0x400 #define _PAGE_TYPE_NONE 0x401 #define _PAGE_TYPE_SWAP 0x403 #define _PAGE_TYPE_FILE 0x601 /* bit 0x002 is used for offset !! */ #define _PAGE_TYPE_RO 0x200 #define _PAGE_TYPE_RW 0x000 #define _PAGE_TYPE_EX_RO 0x202 #define _PAGE_TYPE_EX_RW 0x002 The types where we have no free bits are _PAGE_TYPE_SWAP and _PAGE_TYPE_FILE. That should be true for all architectures, since any free bit could be used to increase the allowable size of the swap device and the file-page offset. For 64 bit we could even make room for another bit in the swap and file ptes since it won't hurt much to lower the swap size / file-page offset. For 31 bit the bits are in short supply. In ESA mode another 3 bits of the pte are reserved, they have to be zero or bad things will happen (specification exceptions). So with the two hardware and the two software bits a total of 7 bits of 32 bits are lost. As the comment for _PAGE_TYPE_FILE indicates I was able to squeeze one more bit out of a pte for file ptes. Which makes 25 free bits for a swap pte and 26 free bits for a file pte, or 32 4GB swap devices and 64GB max file size for remap_file_pages. Again this is ONLY for 31 bit and ONLY for the swap and file ptes. For pte_present() ptes we have 6 free bits. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. - To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
