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Miller" , Linux Memory Management List , netdev@vger.kernel.org, Linux parisc List , Andrew Morton References: <87zfawvt2f.fsf@toke.dk> <87y0qerbld.fsf@toke.dk> Content-Language: en-US From: Helge Deller Autocrypt: addr=deller@gmx.de; keydata= xsFNBF3Ia3MBEAD3nmWzMgQByYAWnb9cNqspnkb2GLVKzhoH2QD4eRpyDLA/3smlClbeKkWT HLnjgkbPFDmcmCz5V0Wv1mKYRClAHPCIBIJgyICqqUZo2qGmKstUx3pFAiztlXBANpRECgwJ r+8w6mkccOM9GhoPU0vMaD/UVJcJQzvrxVHO8EHS36aUkjKd6cOpdVbCt3qx8cEhCmaFEO6u CL+k5AZQoABbFQEBocZE1/lSYzaHkcHrjn4cQjc3CffXnUVYwlo8EYOtAHgMDC39s9a7S90L 69l6G73lYBD/Br5lnDPlG6dKfGFZZpQ1h8/x+Qz366Ojfq9MuuRJg7ZQpe6foiOtqwKym/zV dVvSdOOc5sHSpfwu5+BVAAyBd6hw4NddlAQUjHSRs3zJ9OfrEx2d3mIfXZ7+pMhZ7qX0Axlq Lq+B5cfLpzkPAgKn11tfXFxP+hcPHIts0bnDz4EEp+HraW+oRCH2m57Y9zhcJTOJaLw4YpTY GRUlF076vZ2Hz/xMEvIJddRGId7UXZgH9a32NDf+BUjWEZvFt1wFSW1r7zb7oGCwZMy2LI/G aHQv/N0NeFMd28z+deyxd0k1CGefHJuJcOJDVtcE1rGQ43aDhWSpXvXKDj42vFD2We6uIo9D 1VNre2+uAxFzqqf026H6cH8hin9Vnx7p3uq3Dka/Y/qmRFnKVQARAQABzRxIZWxnZSBEZWxs 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 PIjMNFpK 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>>>>>> Commit ee62ce7a1d90 ("page_pool: Track DMA-mapped pages and unmap t= hem when >>>>>> destroying the pool") changed PP_MAGIC_MASK from 0xFFFFFFFC to 0xc0= 00007c on >>>>>> 32-bit platforms. >>>>>> >>>>>> The function page_pool_page_is_pp() uses PP_MAGIC_MASK to identify = page pool >>>>>> pages, but the remaining bits are not sufficient to unambiguously i= dentify >>>>>> such pages any longer. >>>>> >>>>> Why not? What values end up in pp_magic that are mistaken for the >>>>> pp_signature? >>>> >>>> As I wrote, PP_MAGIC_MASK changed from 0xFFFFFFFC to 0xc000007c. >>>> And we have PP_SIGNATURE =3D=3D 0x40 (since POISON_POINTER_DELTA is = zero on 32-bit platforms). >>>> That means, that before page_pool_page_is_pp() could clearly identify= such pages, >>>> as the (value & 0xFFFFFFFC) =3D=3D 0x40. >>>> So, basically only the 0x40 value indicated a PP page. >>>> >>>> Now with the mask a whole bunch of pointers suddenly qualify as being= a pp page, >>>> just showing a few examples: >>>> 0x01111040 >>>> 0x082330C0 >>>> 0x03264040 >>>> 0x0ad686c0 .... >>>> >>>> For me it crashes immediately at bootup when memblocked pages are han= ded >>>> over to become normal pages. >>>> >>> >>> I tried to take a look to double check here and AFAICT Helge is correc= t. >>> >>> Before the breaking patch with PP_MAGIC_MASK=3D=3D0xFFFFFFFC, basicall= y >>> 0x40 is the only pointer that may be mistaken as a valid pp_magic. >>> AFAICT each bit we 0 in the PP_MAGIC_MASK (aside from the 3 least >>> significant bits), doubles the number of pointers that can be mistaken >>> for pp_magic. So with 0xFFFFFFFC, only one value (0x40) can be >>> mistaken as a valid pp_magic, with 0xc000007c AFAICT 2^22 values can >>> be mistaken as pp_magic? >>> >>> I don't know that there is any bits we can take away from >>> PP_MAGIC_MASK I think? As each bit doubles the probablity :( >>> >>> I would usually say we can check the 3 least significant bits to tell >>> if pp_magic is a pointer or not, but pp_magic is unioned with >>> page->lru I believe which will use those bits. >> >> So if the pointers stored in the same field can be any arbitrary value, >> you are quite right, there is no safe value. The critical assumption in >> the bit stuffing scheme is that the pointers stored in the field will >> always be above PAGE_OFFSET, and that PAGE_OFFSET has one (or both) of >> the two top-most bits set (that is what the VMSPLIT reference in the >> comment above the PP_DMA_INDEX_SHIFT definition is alluding to). >> >=20 > I see... but where does the 'PAGE_OFFSET has one (or both) of the two > top-most bits set)' assumption come from? Is it from this code? >=20 > /* > * PAGE_OFFSET -- the first address of the first page of memory. > * When not using MMU this corresponds to the first free page in > * physical memory (aligned on a page boundary). > */ > #ifdef CONFIG_MMU > #ifdef CONFIG_64BIT > .... > #else > #define PAGE_OFFSET _AC(0xc0000000, UL) > #endif /* CONFIG_64BIT */ > #else > #define PAGE_OFFSET ((unsigned long)phys_ram_base) > #endif /* CONFIG_MMU */ >=20 > It looks like with !CONFIG_MMU we use phys_ram_base and I'm unable to > confirm that all the values of this have the first 2 bits set. I > wonder if his setup is !CONFIG_MMU indeed. Btw, on 32-bit parisc we have PAGE_OFFSET =3D 0x10000000. Other architectures seem to have other values than 0xc0000000 too. Helge