On Apr 4, 2025, at 17:01, Christoph Hellwig hch@lst.de wrote:
After the btrfs compressed bio discussion I think the hugetlb changes that skip the tail pages are fundamentally unsafe in the current kernel.
That is because the bio_vec representation assumes tail pages do exist, so as soon as you are doing direct I/O that generates a bvec starting beyond the present head page things will blow up. Other users of bio_vecs might do the same, but the way the block bio_vecs are generated are very suspect to that. So we'll first need to sort that out and a few other things before we can even think of enabling such a feature.
I would like to express my gratitude to Christoph for including me in the thread. I have carefully read the cover letter in [1], which indicates that an issue has arisen due to the improper use of `vmap_pfn()`. I'm wondering if we could consider using `vmap()` instead. In the HVO scenario, the tail struct pages do **exist**, but they are read-only. I've examined the code of `vmap()`, and it appears that it only reads the struct page. Therefore, it seems feasible for us to use `vmap()` (I am not a expert in udmabuf.). Right?
[1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b...
Thanks, Muchun.