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Miller" , Linux Memory Management List , netdev@vger.kernel.org, Linux parisc List , Andrew Morton References: <87zfawvt2f.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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 lDPdYHqP 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function page_pool_page_is_pp() uses PP_MAGIC_MASK to identify page= pool >> pages, but the remaining bits are not sufficient to unambiguously ident= ify >> such pages any longer. >=20 > 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 handed over to become normal pages. > As indicated by the comment above the definition of the PP_DMA_INDEX_* > definitions, I did put some care into ensuring that the values would not > get mistaken, see specifically this: >=20 > (...) On arches where POISON_POINTER_DELTA is > * 0, we make sure that we leave the six topmost bits empty, as that gu= arantees > * we won't mistake a valid kernel pointer for a value we set, regardle= ss of the > * VMSPLIT setting. >=20 > So if that does not hold, I'd like to understand why not? Because on 32-bit arches POISON_POINTER_DELTA is zero, and as such you basically can't take away any of the remaining low 32 (30) bits. =20 >> So page_pool_page_is_pp() now sometimes wrongly reports pages as page p= ool >> pages and as such triggers a kernel BUG as it believes it found a page = pool >> leak. >> >> There are patches upcoming where page_pool_page_is_pp() will not depend= on >> PP_MAGIC_MASK and instead use page flags to identify page pool pages. U= ntil >> those patches are merged, the easiest temporary fix is to disable the c= heck >> on 32-bit platforms. >=20 > As Jesper pointed out, we also use this check internally in the network > stack, and the patch as proposed will at least trigger the > DEBUG_NET_WARN_ON_ONCE() in include/net/netmem.h. Interestingly it did not triggered this warning for me. Need to look into this. > I think a better > solution would be, as Jesper also alludes to, simply adding more bits to > the mask. For instance, the patch below reserves (somewhat arbitrarily) > six bits instead of two, changing the mask to 0xfc00007c; would that > work? That just shifting the main problem and you may be lucky for short time. page_pool_page_is_pp() need to *reliably* detect pp pages, all other is gu= essing. I don't believe there is any way to really fix it for 32-bit other than reverting your change, or to disable the check (on 32-bit). Helge >=20 > -Toke >=20 > diff --git i/include/linux/mm.h w/include/linux/mm.h > index 1ae97a0b8ec7..17cb8157ba08 100644 > --- i/include/linux/mm.h > +++ w/include/linux/mm.h > @@ -4159,12 +4159,12 @@ int arch_lock_shadow_stack_status(struct task_st= ruct *t, unsigned long status); > * since this value becomes part of PP_SIGNATURE; meaning we can just = use the > * space between the PP_SIGNATURE value (without POISON_POINTER_DELTA)= , and the > * lowest bits of POISON_POINTER_DELTA. On arches where POISON_POINTER= _DELTA is > - * 0, we make sure that we leave the two topmost bits empty, as that gu= arantees > + * 0, we make sure that we leave the six topmost bits empty, as that gu= arantees > * we won't mistake a valid kernel pointer for a value we set, regardl= ess of the > * VMSPLIT setting. > * > * Altogether, this means that the number of bits available is constra= ined by > - * the size of an unsigned long (at the upper end, subtracting two bits= per the > + * the size of an unsigned long (at the upper end, subtracting six bits= per the > * above), and the definition of PP_SIGNATURE (with or without > * POISON_POINTER_DELTA). > */ > @@ -4175,8 +4175,8 @@ int arch_lock_shadow_stack_status(struct task_stru= ct *t, unsigned long status); > */ > #define PP_DMA_INDEX_BITS MIN(32, __ffs(POISON_POINTER_DELTA) - PP_DMA= _INDEX_SHIFT) > #else > -/* Always leave out the topmost two; see above. */ > -#define PP_DMA_INDEX_BITS MIN(32, BITS_PER_LONG - PP_DMA_INDEX_SHIFT - = 2) > +/* Always leave out the topmost six; see above. */ > +#define PP_DMA_INDEX_BITS MIN(32, BITS_PER_LONG - PP_DMA_INDEX_SHIFT - = 6) > #endif > =20 > #define PP_DMA_INDEX_MASK GENMASK(PP_DMA_INDEX_BITS + PP_DMA_INDEX_SHI= FT - 1, \ >=20