netfs_write_begin(9)

🌸 NAME 🌸

netfs_write_begin — a helper to prepare for writing, bestie… but it is [DEPRECATED] 🫠 (more on that in a sec). This page lives in the Kernel API Manual, so you know it's serious kernel business with a side of chaos. 💅

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✨ SYNOPSIS ✨

int netfs_write_begin (struct netfs_inode *ctx , struct file *file ,
       struct address_space *mapping , loff_t pos , unsigned int len , struct
       folio **_folio , void **_fsdata );

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💅 ARGUMENTS 💅

• ctx — the netfs context, aka the vibe container for this whole operation 💖
• file — the file to read from. yes, read from, even though we're beginning a write, iykyk 🌈
• mapping — the mapping to read from 📚
• pos — the file position at which the write will begin 📍
• len — the length of the write, and lowkey it may extend beyond the end of the folio chosen 👀
• _folio — where to put the resultant folio 🎀
• _fsdata — the place for the netfs to store a cookie 🍪

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🧚 DESCRIPTION 🧚

Okay so here's the tea: this helper pre-reads data for a write-begin request by drawing data from the cache if possible, or from the netfs if not. Space beyond the EOF is zero-filled. And multiple I/O requests from different sources will get munged together — yes, literally munged together 😭.

The calling netfs must provide a table of operations, and only one of them, issue_read, is mandatory. Periodt.

The check_write_begin operation can be provided to check for and flush conflicting writes once the folio is grabbed and locked. It is passed a pointer to the fsdata cookie that gets returned to the VM to be passed to write_end. It is permitted to sleep 😴. It should return 0 if the request should go ahead, or it may return an error. It may also unlock and put the folio, provided it sets *foliop to NULL — in which case a return of 0 will cause the folio to be re-got and the process to be retried. Iconic retry behavior, honestly 🔁.

The calling netfs must initialise a netfs context contiguous to the vfs inode before calling this. No skipping that step, bestie 🫶.

This is usable whether or not caching is enabled. Flexible queen behavior ✨.

⚠️ Note that this should be considered deprecated and netfs_perform_write used instead. Like, for real for real — use the other one 💀.

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🔥 SEE ALSO 🔥

• Kernel file ./fs/netfs/buffered_read.c
• netfs_readahead(9)
• netfs_read_folio(9)
• netfs_buffered_read_iter(9)
• netfs_file_read_iter(9)

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xoxo, your kernel bestie 💖 — source/version: ./fs/netfs, dated October 2026 🎀✨