On 03.05.24 03:21, Prakash Sangappa wrote:
This patch proposes to fix hugetlbfs mmap behavior so that the file size does not get updated in the mmap call. The current behavior is that hugetlbfs file size will get extended by a PROT_WRITE mmap(2) call if mmap size is greater then file size. This is not normal filesystem behavior. There seem to have been very little discussion about this. There was a patch discussion[1] a while back, implying hugetlbfs file size needs extending because of the hugetlb page reservations. Looks like this was not merged. It appears there is no correlation between file size and hugetlb page reservations. Take the case of PROT_READ mmap, where the file size is not extended even though hugetlb pages are reserved. On the other hand ftruncate(2) to increase a file size does not reserve hugetlb pages. Also, mmap with MAP_NORESERVE flag extends the file size even though hugetlb pages are not reserved. Hugetlb pages get reserved(if MAP_NORESERVE is not specified) when the hugeltbfs file is mmapped, and it only covers the file's offset,length range specified in the mmap call. Issue: Some applications would prefer to manage hugetlb page allocations explicity with use of fallocate(2). The hugetlbfs file would be PROT_WRITE mapped with MAP_NORESERVE flag, which is accessed only after allocating necessary pages using fallocate(2) and release the pages by truncating the file size. Any stray access beyond file size is expected to generate a signal. This does not work properly due to current behavior which extends file size in mmap call.
Would a simple workaround be to mmap(PROT_READ) and then mprotect(PROT_READ|PROT_WRITE)?
I know, not perfect, but certainly better than mount options? -- Cheers, David / dhildenb
