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U2FsdGVkX1989zK8vTroFqMR1jzTj5Z0Etso//OfvRwFSEX/oGbh9tiF24SVN5M3TDhM22a4O3HR7AtfV5Q1d0O1VG6clbJDrEl8hv9SXhdiIKhnwBT7tK8CYr+QHsu4ixGYaFo4O8atBnKlov7Oum0NTLR4C7CTTAWL0PyqaPthK9p7+6Vy/A5g9GhupsIAaZkos01HBzL57tkcaXAbwFXHJv+b7SMUM9nV4ZabIfdPcV9CKqocDD1hwVD/ZmzxCwLRBsFBG6ar6sahFgpYNNcHVEgyFZfp8R1vJBnqUNkw33KLG+LtBPsnmJ8SkKNPWVz+K9lTkLIfkls0++6fsEErRjxNob8tcaBjy5U91bhWKNwfPGejAzOPjpcZItNwJQZ0I6ikOmewcf8xGrbAJRad/vgKljNMCzncjImb1US1nikwvuY2gtf16j8M4mGs66ybj6vd6GFtoTzukmMOkNAtqo8vOXCe2icPxgJ8drYxlbWgcmBH9mCsYTl6sx73XnVLLbHb1RatX+0JfBaPuzl1O6Jvr6D0s/SILRaMxxFkPKzYTs7M4zFwmcz75KkjGaMB0cHUGUedLoXAk7y7R52UZ/5WnyYnzL3FuU6+eqQ03AEZ4BxtFwSe6OSMttflYZHyHRPCCkTxuuHLgfkG09aUs8UBWFJpDguWFB7UOnRwF+dNTnmg9kfhHBRdLEcMRnSxTZvsJMBHjWaHp/bkk00dyfGLH/GebDeJS1hAGvbNjLJJVMbkvaui5k+B/FgQ6Dc6rkcccq//uDEItr2vbYjKPAm2YkQosK1s3ZlFf5TF7jpDslZQlYWRoyua79lZKpB6YUzbR3IO8Kc16Zi2hGq+cZOZZOIlHuKe1nC3XtFIujUwOqgvE5szgc2MKbMpG+IXaNELmtdoyWft4IvgFeEamb9Ud4AOv3/CSNFZCdLU2IScEw4J/hGLezOC6i0C34Mzvj3jcJn48Op6cH+ 4HRuSAnD 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 17 Aug 2026, at 11:06, Alan Stern wrote: > On Mon, Aug 17, 2026 at 10:37:29AM -0400, Zi Yan wrote: >> On 17 Aug 2026, at 10:34, Zi Yan wrote: >>> But the warning here is when kernel user wants buddy allocator to giv= e >>> what it cannot allocate, a page order > MAX_PAGE_ORDER. The warning >>> tells that kernel user please ask for a reasonably sized memory. > > That's fine, but it doesn't have to be done in a way that will crash > many systems. > >>> The issue here is that the inode.c code passes the user input len wit= hout >>> checking to page allocator code. Capping that is a minimal requiremen= t >>> to prevent untrusted userspace input getting into trusted kernel spac= e code >>> easily. > > I disagree. If the memory allocators are so fragile that userspace can= > break into the kernel just by asking for too much memory, the allocator= s > should be fixed. > > Furthermore, it's generally recognized that library routines such as > kmalloc() should check their own inputs rather than relying on their > callers to do this work for them. > >>>>> Why are we emitting a WARN if an allocation fails, given that this = will >>>>> often panic the kernel? Should we on the core MM side dial that ba= ck >>>>> to a pr_warn() and a helpful backtrace? >>>> >>>> I think that would be a very good idea. Only the caller knows wheth= er >>>> an allocation failure will leave the system in an unstable state; th= e >>>> library routine shouldn't try to make this decision on its own. >>> >>> In this case, the WARN is emitted not because of an allocation failur= e, >>> but an invalid input to buddy allocator (order > MAX_PAGE_ORDER). The= >>> WARN is for kernel developers, telling them their code is asking too = much >>> free memory and core MM cannot handle it. Suppressing that means >>> code outside MM can abuse page allocator. Code like doing >>> alloc_pages(MAX_PAGE_ORDER + 1, __GFP_NOFAIL | __GFP_NOWARN) should n= ot >>> exist, instead of just getting pr_warn() and failures. > > Again, I disagree with some of the details of this argument in this > context. For instance, if all kernel developers are supposed to know > that they shouldn't ask kmalloc() for more than MAX_PAGE_ORDER at the > risk of provoking a WARN, if this is such an important restriction, the= n > shouldn't this requirement be mentioned in the kerneldoc for kmalloc()?= kmalloc is the normal method of allocating memory for objects smaller than page size in the kernel. See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tr= ee/include/linux/slab.h?h=3Dv7.2#n1001 In this case, kmalloc is used to request > 4MB memory. > > Regardless, if it is important to let kernel developers know that their= > code is doing something wrong, why not make the WARN conditional on > CONFIG_EXPERT or something similar? In other words, prevent it from > crashing production systems. Best Regards, Yan, Zi